THE  ANALYSIS  OF 
RUBBER 


BY 

JOHN  B.  TUTTLE 

A I 


American  Chemical  Society 
Monograph  Series 


BOOK  DEPARTMENT 
The  CHEMICAL  CATALOG  COMPANY,  Inc. 

19  EAST  24TH  STKEET,  NEW  YOBK,  U.  S.  A. 
1922 


Coi'vnu.HT,  1922,  BY 

77,,    CHKM1CAL  CATALOG  COMPANY,  Inc. 
All  Rights  Reserved 


GENERAL   INTRODUCTION 

American    Chemical    Society    Series    of 
Scientific  and  Technologic  Monographs 

By  arrangement  with  the  Interallied  Conference  of  Pure  and 
Applied  Chemistry,  which  met  in  London  and  Brussels  in  July, 
1919,  the  American  Chemical  Society  was  to  undertake  the  pro- 
duction and  publication  of  Scientific  and  Technologic  Mono- 
graphs on  chemical  subjects.  At  the  same  time  it  was  agreed 
that  the  National  Research  Council,  in  cooperation  with  the 
American  Chemical  Society  and  the  American  Physical  Society, 
should  undertake  the  production  and  publication  of  Critical 
Tables  of  Chemical  and  Physical  Constants.  The  American 
Chemical  Society  and  the  National  Research  Council  mutually 
agreed  to  care  for  these  two  fields  of  chemical  development. 
The  American  Chemical  Society  named  as  Trustees,  to  make  the 
necessary  arrangements  for  the  publication  of  the  monographs, 
Charles  L.  Parsons,  Secretary  of  the  American  Chemical  So- 
ciety, Washington,  D.  C.;  John  E.  Teeple,  Treasurer  of  the 
American  Chemical  Society,  New  York  City;  and  Professor 
Gellert  Alleman  of  Swarthmore  College.  The  Trustees  have  ar- 
ranged for  the  publication  of  the  American  Chemical  Society 
series  of  (a)  Scientific  and  (b)  Technologic  Monographs  by  the 
Chemical  Catalog  Company  of  New  York  City. 

The  Council,  acting  through  the  Committee  on  National 
Policy  of  the  American  Chemical  Society,  appointed  the  editors, 
named  at  the  close  of  this  introduction,  to  have  charge  of  secur- 
ing authors,  and  of  considering  critically  the  manuscripts  pre- 
pared. The  editors  of  each  series  will  endeavor  to  select  topics 
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authorities  in  their  respective  fields.  The  list  of  monographs  thus 
far  secured  appears  in  the  publisher's  own  announcement  else- 
where in  this  volume. 

The  development  of  knowledge  in  all  branches  of  science,  and 

3 


»*y  r* 


4  GENERAL  INTRODUCTION 

especially  in  chemistry,  has  been  so  rapid  during  the  last  fifty 
years  and  the  iields  covered  by  this  development  have  been  so 
varied  that  it  is  diilicult  for  any  individual  to  keep  in  touch  with 
the  progress  in  branches  of  science  outside  his  own  specialty.  In 
spite  of  the  facilities  for  the  examination  of  the  literature  given 
by  Chemical  Abstracts  and  such  compendia  as  Beilstein's  Hand- 
budi  der  Organischen  Chemie,  Richter's  Lexikon,  Ostwald's  Lehr- 
buch  der  Allgemeinen  Chemie,  Abegg's  and  Gmelin-Kraut's 
llandbuch  der  Anorganischen  Chemie  and  the  English  and 
French  Dictionaries  of  Chemistry,  it  often  takes  a  great  deal  of 
time  to  coordinate  the  knowledge  available  upon  a  single  topic. 
Consequently  when  men  who  have  spent  years  in  the  study  of 
important  subjects  are  willing  to  coordinate  their  knowledge  and 
present  it  in  concise,  readable  form,  they  perform  a  service  of 
the  highest  value  to  their  fellow  chemists. 

it  was  with  ,°  clear  recognition  of  the  usefulness  of  reviews 
of  this  character  that  a  Committee  of  the  American  Chemical 
Society  recommended  the  publication  of  the  two  series  of  mono- 
graphs under  the  auspices  of  the  Society. 

Two  rather  distinct  purposes  are  to  be  served  by  these  mono- 
graphs. The  first  purpose,  whose  fulfilment  will  probably  ren- 
der to  chemists  in  general  the  most  important  service,  is  to  pre- 
sent the  knowledge  available  upon  the  chosen  topic  in  a  readable 
form,  intelligible  to  those  whose  activities  may  be  along  a  wholly 
different  line.  Many  chemists  fail  to  realize  how  closely  their 
investigations  may  be  connected  with  other  work  which  on  the 
surface  appears  far  afield  from  their  owrn.  These  monographs 
will  enable  such  men  to  form  closer  contact  with  the  work  of 
chemists  in  other  lines  of  research.  The  second  purpose  is  to  pro- 
mote research  in  the  branch  of  science  covered  by  the  mono- 
graph, by  furnishing  a  well  digested  survey  of  the  progress 
already  made  in  that  field  and  by  pointing  out  directions  in  which 
investigation  needs  to  be  extended.  To  facilitate  the  attainment 
of  this  purpose,  it  is  intended  to  include  extended  references  to 
the  literature,  which  will  enable  anyone  interested  to  follow  up 
the  subject  in  more  detail.  If  the  literature  is  so  voluminous  that 
a  complete  bibliography  is  impracticable,  a  critical  selection  will 
be  made  of  those  papers  which  are  most  important. 

The  publication  of  these  books  marks  a  distinct  departure  in 
the  policy  of  the  American  Chemical  Society  inasmuch  as  it  is 


GENERAL  INTRODUCTION  5 

a  serious  attempt  to  found  an  American  chemical  literature  with- 
out primary  regard  to  commercial  considerations.  The  success 
of  the  venture  will  depend  in  large  part  upon  the  measure  of 
cooperation  which  can  be  secured  in  the  preparation  of  books 
dealing  adequately  with  topics  of  general  interest;  it  is  earnestly 
hoped,  therefore,  that  every  member  of  the  various  organizations 
in  the  chemical  and  allied  industries  will  recognize  the  importance 
of  the  enterprise  and  take  sufficient  interest  to  justify  it. 


AMERICAN     CHEMICAL    SOCIETY 

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JULIUS  STIEGLITZ.  ARTHUR  D.  LITTLE, 

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The  Corrosion  of  Alloys.    By  C.  G.  Fink. 
Piezo-Chemistry.    By  L.  H.  Adams. 
Cyanamide.    By  Joseph  M.  Braham. 
Liquid  Ammonia  as  a  Solvent.    By  E.  C.  Franklin. 
Wood  Distillation.    By  L.  F.  Hawley. 
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The  Animal  as  a  Converter.    By  H.  P.  Armsby  and  C.  Robert 

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Glue  and  Gelatin.    By  Jerome  Alexander. 
Organic  Arsenical  Compounds.    By  George  W.  Raiziss  and  Jos. 

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Valence,  and  the  Structure  of  Atoms  and  Molecules.    By  Gil- 
bert N.  Lewis. 

Shale  Oil.    By  Ralph  H.  McKee. 

Aluminothermic  Reduction  of  Metals.    By  B.  D.  Saklatwalla. 
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PREFACE 

The  tendency  of  the  industry  is  towards  simplification  of 
methods  and  materials.  The  readjustment  of  conditions  to  the 
basis  of  an  adequate  supply  of  crude  rubber — a  condition  which 
did  not  obtain  twenty  years  ago — has  by  the  operation  of  natural 
economic  laws  eliminated  from  general  use  many  pigments,  rub- 
ber substitutes,  and  low  grades  of  rubber.  These  are  not  likely  to 
return,  and  we  may  dismiss  them  from  consideration,  confining 
ourselves  to  the  materials  found  in  the  every  day  life  of  the 
industry  as  it  exists  today. 

In  any  work  on  analysis,  and  especially  on  industrial  sub- 
stances, it  is  impossible  to  avoid  the  presentation  of  the  subject 
from  a  very  personal  point  of  view.  Many  methods,  and  modifi- 
cations of  methods,  are  written  on  a  single  phase  of  the  analysis, 
with  a  great  variety  of  purposes  back  of  them.  In  the  analysis 
of  rubber,  methods  have  been  published  because  they  were  shorter 
than  existing  ones;  some  used  less  expensive  materials,  or  more 
simple  equipment ;  and  some  because  they  really  were  an  improve- 
ment. Few  of  these  methods  were  thoroughly  developed  before 
publication ;  the  user  must  discover  for  himself  the  limits  of  error 
and  applicability.  It  is  usually  safer  to  hold  fast  to  such  methods 
as  have  stood  the  test  of  time,  and  whenever  there  may  be  any 
methods  for  any  part  of  a  rubber  analysis  which  are  not  included 
herewith,  it  is  because  data  are  lacking  as  to  their  ability  to 
accomplish  the  desired  purpose.  The  omission  does  not  imply 
lack  of  merit,  but  merely  that  sufficient  experimental  evidence  is 
not  yet  forthcoming  to  warrant  an  unqualified  approval. 

Primarily,  this  monograph  is  addressed  to  the  chemists  in  the 
consumers'  laboratories,  and  to  those  who,  without  any  previous 
experience  in  the  technology  or  analysis  of  rubber,  may  be  called 
upon  to  deal  with  a  problem  in  which  the  composition  of  rubber 
may  play  a  more  or  less  important  part.  Nevertheless,  it  is  the 
author's  hope  that  it  may  not  come  amiss  to  those  colleagues 
comprizing  the  technical  staff  of  the  laboratories  of  the  manu- 
facturing plants,  who  may  find  it  desirable  to  study  a  competi- 

7 


8  PREFACE 

tor's  product,  or  who  may  be  required  to  produce  materials  to 
accord  with  the  consumers'  specifications. 

In  view  of  the  probability  of  this  monograph  reaching  chemists 
of  limited  experience  in  the  technology  of  rubber,  Appendix  A, 
on  the  methods  of  preparation  of  rubber  compounds,  and  Ap- 
pendix B,  on  the  physical  testing  of  rubber,  have  been  added. 
These  appendices  are  necessarily  elemental  in  character,  but  they 
may  serve  as  connecting  links  between  these  subjects,  and  the 
chemistry  of  the  analysis  of  rubber. 

J.  B.  T. 


TABLE  OF  CONTENTS 

PAGE 

PREFACE  7 


CHAPTER 


I  THE  PURPOSE  OF  RUBBER  ANALYSIS;  WHAT  Is  RUB- 
BER; THE  NEED  FOR  CHEMICAL  ANALYSIS  OF 
RUBBER 11 

II  THE  COMPOSITION  OF  CRUDE  RUBBER;  CONSTITU- 
ENTS OF  CRUDE  RUBBER  OTHER  THAN  THE 
RUBBER  HYDROCARBONS 16 

III  THE  PREPARATION  OF  RUBBER  COMPOUNDS;  CRUDE 

RUBBER;  RECLAIMED  RUBBER;  OIL  SUBSTITUTES; 
MINERAL  RUBBER  ;  MINERAL  HYDROCARBONS  ;  OILS 
AND  WAXES;  VULCANIZING  MATERIALS;  ORGANIC 
ACCELERATORS;  INORGANIC  ACCELERATORS;  INOR- 
GANIC FILLERS 26 

IV  THEORY  OF  VULCANIZATION;  COLD  VULCANIZATION; 

VULCANIZATION  WITH  MIXED  GASES;  OSTRO- 
MUISLENSKIl's  THEORIES  OF  VULCANIZATION  .  57 

V    SAMPLING 64 

VI  EXTRACTIONS;  ACETONE  EXTRACT;  CHLOROFORM  EX- 
TRACT; ALCOHOLIC  POTASH  EXTRACT;  ANALYSIS 
OF  THE  ACETONE  EXTRACT  .......  68 

VII  THE  DETERMINATION  OF  RUBBER;  THE  TETRABRO- 
MIDE  METHOD;  THE  NITROSITE  METHOD;  THE 
INDIRECT  METHODS;  DIFFERENCE  METHODS  .  .  76 

VIII  SULFUR  DETERMINATIONS;  TOTAL  SULFUR;  FREE 
SULFUR;  SULFUR  OF  VULCANIZATION;  SULFUR  IN 
FILLERS 84 

IX    DETECTION  OF  ORGANIC  ACCELERATORS  94 


10  TABLE  OF  CONTENTS 

CHAPTER  PAGB 

X    MINERAL     ANALYSIS;     SPECIAL     DETERMINATIONS; 

SPECIFIC  GRAVITY 97 

XI    MICRO-SECTIONING  AND  MICROPHOTOGRAPHY  .     .     .  114 

XII    CALCULATION  TO  APPROXIMATE  FORMULAS     .     .     .  117 

BIBLIOGRAPHY 121 

APPENDIX  A    PREPARATION    OF    MATERIALS    FOR    RUBBER 

MANUFACTURE 139 

"         B    PHYSICAL  TESTS 143 

C    TABLE  OF  SPECIFIC  GRAVITIES 149 

INDEX 153 


THE  ANALYSIS  OF  RUBBER 

Chapter  I. 
The  Purpose  of  Eubber  Analysis. 

The  growth  of  the  rubber  industry  has  been  tremendous,  espe- 
cially, so  far  as  volume  is  concerned,  since  the  advent  of  the 
pneumatic  tire.    More  than  any  one  other  cause,  the  resiliency 
afforded  by  the  pneumatic  bicycle  tire  was  responsible  for  the 
wide  spread  popularity  of  the  bicycle,  and  the  rubber  automobile 
tire  has  played  an  equally,  if  not  more  important  role  in  the 
development  of  motor  driven  vehicles.    In  the  production  of  the 
various  rubber  articles,  besides  the  essential  rubber  and  sulfur 
which  make  up  the  vulcanized  rubber,  we  need  a  vast  volume  of 
pigments  or  fillers,  because  by  their  use  we  may  modify  the 
properties  of  the  vulcanized  rubber  so  as  to  attain  a  degree  of 
service  which  would  otherwise  be  impossible.     We  must  not, 
therefore,  look  upon  these  added  substances  as  adulterants,  or 
even  mere  diluents,  but  as  integral  parts  of  the  whole,  for  by 
their  service  they  have  earned  the  right  to  due  consideration. 
Some,  it  is  true,  are  largely  valuable,  owing  to  the  fact  that  they 
lower  the  cost  of  the  products  in  which  they  are  included.    The 
rubber  industry  makes  big  demands  upon  the  producers  of  raw 
materials,  such  as  zinc  oxide  and  sulfide,  lead  compounds,  carbon 
black,  magnesium  oxide,  and  talc,  and  were  we  forced  to  depend 
upon  these  pigments  alone,  the  costs  would  soon  rise  to  prohibi- 
tive heights,  with  concomitant  injury  not  merely  to  the  rubber 
industry,  but  to  others  as  well,  such  as  paints  and  inks,  which 
depend  for  their  existence  upon  an  adequate  supply  of  these  same 
pigments.    We  are  thus  doubly  obliged  to  seek  far  and  wide  for 
new  materials  which  will  accomplish  one  of  two  things:  produce 
the  same  or  even  better  quality  at  a  lower  cost,  or  a  better 
quality  at  the  same  cost. 

In  every  line,  there  is  a  more  or  less  clearly  defined  standard 

11 


12  THE  ANALYSIS  OF  RUBBER 

of  service,  and  the  future  of  the  industry  is  quite  definitely  tied 
up  with  the  results  obtained  in  the  present  by  the  attainment  of 
that  standard.  In  order  to  use  correctly  any  material,  it  is 
important  that  we  know  the  degree  of  purity  of  the  commercial 
grades,  the  influence  of  possible  impurities  upon  the  quality  of  the 
products,  and,  most  important  of  all,  the  degree  of  uniformity 
obtainable  from  one  period  to  another.  These  data  can  be 
secured  only  through  careful  and  persistent  testing  of  the  raw  and 
finished  materials. 

It  is  not  our  purpose  here  to  undertake  a  description  of  the 
functions  and  usage  of  the  various  materials  to  be  mentioned 
later,  but  merely  to  discuss  them  from  the  point  of  view  of  their 
chemical  properties.  For  the  various  other  phases  of  the  subject, 
the  reader  is  referred  to  the  bibliography  which  is  included  here- 
with. 

What  Is  "Rubber"? 

Probably  few  words  in  general  usage  are  applied  as  generally 
as  the  word  rubber.  Strictly  speaking,  the  word  belongs  to  the 
polyterpene  having  the  formula  (C10H16)X.  We  know,  however, 
that  there  is  an  homologous  series  of  these  polymerized  products 
differing  from  each  other  by  the  constant  quantity  2  CH2,  and 
these  products  have  so  many  of  the  qualities  peculiar  to 
(C10H]6)X  that  they  too  are  called  rubber.  Thus  we  may  say  that 
there  is  a  rubber  series  analagous  to  the  paraffin  series,  etc. 
The  planters  who  cultivate  the  plantations  call  their  product  rub- 
ber, or  crude  rubber,  although,  in  addition  to  the  polyterpene, 
there  is  2%  and  upwards  of  acetone-soluble  substances  called 
resins;  2  to  6%  of  nitrogenous  substances,  akin  to  the  proteins; 
and  small  amounts  of  substances  possessing  the  properties  of 
catalysts  of  the  vulcanization  process.  The  manufacturer  pro- 
duces rubber  products,  although  in  addition  to  the  rubber  as 
obtained  from  the  plantations,  many  other  substances  are  added, 
both  organic  and  inorganic,  because  of  certain  qualities  which 
such  additions  produce  in  the  finished  article.  Moreover,  chemi- 
cally speaking,  we  have  in  the  hot  vulcanized  articles  an  entirely 
IH-W  sorios  of  products,  viz.,  the  sulfur  addition  products  of  the 
polyterpene,  a  scries  which  passes  from  the  extreme  of  pure 
rubber  at  one  end  to  a  constant  composition  of  (C10H1CS2)X  at 
the  other  end. 


THE  PURPOSE  OF  RUBBER  ANALYSIS  13 

In  order  to  avoid  confusion,  for  our  own  purposes  we  will  use 
the  term  "rubber"  to  mean  any  mixture  of  (C10H16)X  (its  homo- 
logues  are  negligible  commercially,  at  present)  with  any  other 
substances,  in  either  the  vulcanized  or  unvulcanized  state. 
"Crude  rubber"  will  apply  only  to  the  materials  as  obtained  from 
the  rubber  trees;  and  where  the  polyterpene  itself  is  indicated, 
we  will  use  the  term  "rubber  hydrocarbon."  "Rubber  compound" 
will  be  used  to  indicate  the  formula  of  a  commercial  mixing. 

From  an  analytical  point  of  view,  it  is  of  little  consequence 
whether  the  unit  in  the  molecule  of  rubber  is  C5H8  or  C10H16. 
There  seems  to  be  a  preponderance  of  evidence  in  favor  of  the 
latter;  in  any  event,  we  do  know  that  while  rubber  can  be 
synthesized  from  isoprene,  CH2:C(CH3)  .CH:CH2,  the  rubber 
molecule  itself  contains  only  one  double  bond  for  each  group  of 
C6H8.  By  vulcanizing  rubber  with  a  large  excess  of  sulfur,  C.  0. 
Weber  obtained  a  hard  rubber  corresponding  to  C10HlttS2)x. 
With  bromine,  rubber  has  been  found  to  form  (C10H16Br4)x. 
These  experiments  have  been  repeated  so  many  times,  that  there 
seems  to  be  no  necessity  for  further  argument  as  to  the  existence 
of  the  two  double  bonds.  The  importance  of  this  fact  is  seen 
when  we  realize  that  this  fact  is  the  basis  upon  which  have  been 
built  the  two  classes  of  direct  determinations  of  rubber,  the 
tetrabromide,  and  the  nitrosite,  both  of  which  will  be  discussed 
in  their  proper  place. 

The  Need  for  Chemical  Analysis  of  Rubber. 

Any  scheme  which  may  be  suggested  for  the  analysis  of  rubber, 
either  vulcanized  or  unvulcanized,  must  take  into  consideration 
the  fact  that  there  are  two  groups,  with  widely  differing  points 
of  view,  which  are  interested  in  the  subject.  We  have  first  the 
manufacturers'  chemists,  who  test  their  own  products  to  deter- 
mine what  changes  have  taken  place  during  the  process  of  manu- 
facture; and  their  competitors,  products  to  ascertain  what  the 
latter  are  using.  Analysts  of  this  group  may  use  methods  which 
their  knowledge  of  the  subject  tells  them  will  give  accurate 
results,  even  though  it  is  known  that  such  methods  are  not  uni- 
versally applicable.  The  second  group  embraces  the  consumers, 
who,  as  a  rule,  are  endeavoring  to  learn  whether  or  not  the  article 
complies  with  certain  stipulated  requirements,  or  specifications, 


14  THE  ANALYSIS  OF  RUBBER 

Here  a  definite  procedure  is  obligatory,  for  in  order  to  avoid  dis- 
putes, the  specifications  usually  (and  if  they  do  not,  they 
should) ,J  contain  a  more  or  less  detailed  description  of  the  analyt- 
ical methods.  Since  the  composition  is  unknown,  it  is  clear  that 
the  methods  in  use  by  the  consumers  should  be  as  nearly  uni- 
versally applicable  as  it  is  possible  to  make  them.  From  time  to 
time,  there  have  appeared  suggestions  for  making  analyses  of 
rubber  compounds,  but  too  frequently  the  authors  have  neglected 
to  take  into  consideration  these  different  points  of  view,  and  this 
omission  has  materially  reduced  the  value  of  the  suggestions. 

Rubber  is  a  hydrocarbon  of  the  terpene  family,  existing  in  a 
polymerized  form,  and  having  the  composition  (C10H16)X.  The 
size  of  the  molecule  is  unknown,  although  it  is  believed  to  be  quite 
large,  but  we  do  know  that  each  group  of  C10H16  contains  two 
double  bonds.  Double  bonds  are  unstable,  and  there  is  always  a 
tendency  for  such  double  bonds  to  add  various  elements,  or 
group  of  elements,  which  will  tend  to  produce  a  more  stable 
form.  Thus  we  find  that  the  double  bonds  of  rubber  may  take 
up  oxygen,  ozone,  sulfur,  selenium,  sulfur  chloride,  chlorine, 
bromine,  etc.,  producing  new  chemical  substances  with  distinctly 
new  properties,  many  of  which  are  more  useful  in  a  commercial 
sense  than  the  original  substance.  Industrially,  the  most  im- 
portant of  these  compounds  are  those  formed  by  the  addition  of 
sulfur,  and  sulfur  monochloride,  the  chemical  process  being 
termed  "vulcanization,"  or  "curing."  Crude  rubber  is  a  soft, 
plastic  substance,  soluble  in  naphtha,  benzene,  chloroform,  carbon 
bisulfide,  from  which,  by  the  simple  process  of  evaporation,  it 
may  be  recovered  in  its  original  form.  The  addition  of  compara- 
tively small  amounts  of  sulfur  is  sufficient  to  destroy  the  solu- 
bility in  these  solvents.  Such  vulcanized  compounds  can,  by  pro- 
longed heating,  be  brought  into  solution  in  various  solvents,  but 
there  is  this  distinction,  that,  in  the  latter  case,  the  solution  is 
accompanied  by  a  depolymerization,  and  evaporation  of  the  sol- 

1  There  is  such  a  diversity  of  opinion  concerning  the  best  method  for  any 
single  determination,  ami  since  the  interpretation  of  the  analysis,  rather  than 
the  absolute  results  obtained,  is  the  more  important  of  the  two,  it  must  neces- 
sarily follow  that  the  results  of  the  analysis  are  inseparable  from  the  method 
by  which  they  were  obtained.  It,  is  not  sufficient  merely  to  say  that  a  sample 
has  3.00%  ,,f  sulfur,  it  must  be  stated  that  it  has  3.00%  of  sulfur  when  deter- 
mined by  a  certain  method.  This  has  been  one  of  the  glaring  weaknesses  of 
the  average  specification  in  this  country,  and  has  been  the  cause  of  a  great 
deal  of  controversy  and  actual  financial  loss. 


THE  PURPOSE  OF  RUBBER  ANALYSIS  15 

vent  will  not  give  us  the  rubber  in  the  same  condition  in  which  it 
existed  before  solution.  Rubber  containing  only  a  small  quantity 
of  combined  sulfur  is  tough  and  elastic,  but  as  the  percentage  of 
combined  sulfur  increases,  the  degree  of  extensibility  becomes  less 
and  less,  the  rubber  becomes  harder  until  we  obtain  the  familiar 
substance,  vulcanite,  or  hard  rubber,  and  the  limit  of  sulfur  addi- 
tion is  found  at  (C10H16  S2)x.  To  effect  the  combination  between 
the  rubber  and  sulfur,  catalysts  are  employed,  both  organic  and 
inorganic,  while  to  produce  the  desired  properties  in  the  finished 
article,  various  oils,  waxes,  gums  and  pigments  are  added. 

It  would  seem,  therefore,  to  be  quite  apparent  that  in  order  to 
understand  the  analysis  of  rubber,  one  must  be  familiar  with  the 
materials  which  enter  into  the  rubber  compounds,  the  chemical 
changes  which  take  place  during  vulcanization,  as  well  as  merely 
the  analytical  methods.  In  this  way,  the  analysis  may  be 
directed  towards  bringing  out  the  really  important  points  in  the 
rubber  compound.  The  general  scheme  which  has  been  adopted, 
is  to  first  present  a  description  of  the  raw  materials,  the  methods, 
and  the  theories  of  vulcanization.  This  will  be  followed  by  direc- 
tions for  sampling,  general  and  specific  methods  of  analysis,  and 
finally  some  suggestions  for  interpreting  the  results  of  the  analy- 
sis, with  the  view  to  reconstructing  the  formula  of  the  rubber 
compound. 


Chapter  II. 
The  Composition  of  Crude  Rubber. 

Crude  rubber  is  obtained  from  various  trees,  shrubs,  or  vines. 
Some  of  these  grow  wild,  and  others  are  cultivated  for  the  sake  of 
their  yield  of  rubber.  Twenty  years  ago  cultivated  or  plantation 
rubber  was  practically  unknown;  the  crude  rubber  of  that  time 
was  obtained  from  all  quarters  of  the  tropical  world,  Brazil  fur- 
nishing the  greater  portion  of  the  wild  rubber.  Not  only  did 
Brazil  furnish  the  major  part  of  the  rubber,  but  it  was  also  the 
best  in  quality,  largely  because  of  the  care  taken  in  preparation, 
and  the  uniformity  achieved  in  spite  of  the  rather  crude  methods 
which  were  employed.  One  reason  for  this  uniformity  was  that 
most  of  the  rubber  was  obtained  from  a  single  species,  the  Hevea 
Braziliensis,  which  today  is  not  only  the  source  of  the  best  wild 
rubber,  but,  through  transplanting,  is  also  the  chief,  one  might 
almost  say  the  entire,  source  of  the  plantation  rubber  as  well. 
The  Hevca  rubber  became  known  commercially  as  Para  rubber, 
from  the  port  from  which  shipments  were  made. 

Para  Rubber.  Two  main  subdivisions  are  made  in  Para  rub- 
ber, the  Up-river,  and  Islands.  The  former  comprises  rubber 
collected  in  the  inland  section,  along  the  Amazon  river  and  its 
branches.  The  Islands  rubber  is  so  named  because  it  is  largely 
collected  in  the  islands  of  the  delta  of  the  Amazon,  and  the  ad- 
jacent country.  There  are  subdivisions  of  the  main  group,  the 
Tp-river  including  Acre,  Bolivian,  Madeira,  Manaos,  etc.,  and 
the  Islands  rubber  is  similarly  subdivided. 

Rubber  comes  from  the  latex  of  the  trees,  and  the  latex  is 
gathered  by  making  a  number  of  small  cuts  extending  just  below 
the  bark.  The  latex  flows  from  these  cuts,  and  is  caught  in  small 
nips.  The  rubber  gatherer  collects  the  latex  daily,  takes  it  to  his 
hut.  and  prepares  it  for  the  market  by  the  process  of  coagulation 
and  smoking.  A  paddle  is  dipped  into  the  latex,  and  the  thin 
film  which  remains  on  the  surface  is  coagulated  by  holding  it  over 

16 


THE  COMPOSITION  OF  CRUDE  RUBBER          17 

the  smoke  from  burning  uri-curi  nuts.  The  heat  and  smoke  break 
down  the  emulsion,  separating  the  rubber  from  the  so-called 
serum  of  the  latex.  Much  of  the  serum  drips  out,  but  a  consider- 
able portion  is  retained,  and  the  solids  contained  therein  become 
a  part  of  the  crude  rubber,  profoundly  influencing  the  vulcaniza- 
tion and  the  physical  properties.  The  operation  of  dipping  (or 
the  latex  may  be  poured  over  the  paddle)  and  smoking  is  con- 
tinued until  a  fair  sized  ball  is  obtained.  The  rubber  so  prepared 
is  called  Fine  Para,  but  if  for  any  reason  fermentation  or  oxida- 
tion should  set  in,  and  the  rubber  become  sticky,  it  is  classed  with 
the  lower  grades.  The  scrap  from  the  cups,  buckets,  and  from  the 
bark  of  the  trees,  is  gathered  together,  and  called  "Coarse 
Para."  The  shape  and  general  appearance  of  these  "balls"  varies 
widely,  but  the  method  of  preparation  is  the  same  throughout,  so 
that  there  actually  exists  a  remarkably  uniform  method  of  prep- 
aration throughout  the  entire  territory  where  the  Para  rubber  is 
gathered. 

Castilloa.  Second  among  the  wild  rubbers  is  that  obtained 
from  the  Castilloa  Ulei  or  Castilloa  Elastica  which  produce  the 
kinds  known  as  Caucho,  Centrals,  etc.  This  rubber  is  coagulated 
in  bulk,  is  not  smoked,  and  appears  on  the  market  as  balls, 
sheets,  strips,  or  slabs.  It  is  subdivided  into  grades,  but,  even  in 
the  best,  there  is  nothing  like  the  uniformity  of  quality  which 
one  finds  in  the  Para  rubber. 

African  Rubbers.  African  rubbers  are  largely  gathered  from 
vines,  chiefly  the  Landolphia,  with  innumerable  sorts  and  grades 
many  of  which  are  quite  indistinguishable,  even  to  the  expert,  and 
certainly  cannot  be  identified  after  vulcanization.  Some  African 
rubbers  are  prepared  according  to  methods  peculiar  to  the  place, 
by  means  of  which  they  can  be  identified,  or  they  may  gather 
from  the  means  by  which  they  are  coagulated  an  odor  peculiarly 
their  own,  but  the  differences  from  one  lot  of  the  same  name  to 
the  next  is  frequently  greater  than  that  between  two  entirely 
different  sorts.  During  the  past  year  there  has  been  a  decided 
diminution  in  the  quantity  of  African  rubber  produced,  and  many 
sorts  have  entirely  disappeared  from  the  market  as  their  quality 
is  so  poor  that  the  price  they  will  bring  on  the  present  day 
markets  is  not  sufficient  to  pay  the  cost  of  collecting.  If  the 
Plantation  rubber  continues  to  increase  at  anywhere  near  the 
rate  it  has  for  the  past  eight  or  ten  years,  it  will  mean  such  a  low 


18  THE  ANALYSIS  OF  RUBBER 

standard  market  price  for  the  best  grades  of  rubber  that  the 
poorer  African  sorts  will  disappear  altogether.  From  the  experi- 
ences which  the  manufacturers  have  had  in  trying  to  produce 
uniform  quality  material  with  such  stuff,  we  may  surmise  that 
no  tears  will  be  shed  at  the  loss. 

Guayule.  Guayule  is  the  rubber  obtained  from  the  shrub 
Parthenium  Argentatum,  found  extensively  in  Mexico  and  Texas. 
This  rubber  is  not  obtained  in  the  form  of  a  latex,  but  the* 
plants  are  cut  down,  and  the  rubber  which  exists  in  the  stems, 
leaves,  and  branches  of  the  plant,  is  separated  by  mechanical  or 
chemical  means,  or  both.  The  crude  Guayule  thus  obtained  runs 
very  high  in  resins  and  other  impurities ;  indeed,  these  form  about 
two  thirds  of  the  crude  rubber.  It  usually  undergoes  a  process 
of  purification,  or  deresinification  in  order  to  prepare  it  for  the 
market,  whereby  the  rubber  hydrocarbon  content  is  raised  to 
somewhere  around  75%,  or  even  higher.  Guayule  is  a  soft, 
sticky,  stretchy  rubber,  retaining  these  properties  to  a  high 
degree  even  after  vulcanization,  and  it  finds  its  chief  use  as  a  con- 
stituent of  frictions. 

Pontianak.  Java,  Borneo,  and  the  neighboring  countries,  pro- 
duce a  tree,  the  Dyera  Costidata,  which  yields  a  product  contain- 
ing about  90%  of  resins  and  similar  substances,  and  about  10% 
of  rubber.  This  mixture  is  known  chiefly  as  Pontianak,  or  Jelui 
tong  rubber.  In  the  crude  form,  it  is  quite  hard,  owing  to  the 
high  resin  content,  and  particularly  to  the  nature  of  the  resin. 
In  the  process  of  purification  of  crude  Pontianak,  a  large  part  of 
this  resin  is  removed,  and  is  marketed  separately.  Pontianak 
resin  finds  some  use  in  rubber  mixings;  it  is  hard,  brittle  resin, 
with  a  conchoidal  fracture,  very  much  resembling  our  ordinary 
rosin.  It  is  soluble  in  acetone,  chloroform,  benzene,  and  other 
organic  solvents,  and  consists  largely  of  unsaponifiable  hydrocar- 
bons. Ellis  and  Wells1  find  that  on  heating,  the  solubility  of  the 
resin  and  the  percentage  of  unsaturated  compounds  increase. 
While  there  is  some  demand  commercially  for  this  resin,  it  does 
not  appear  to  be  sufficiently  extensive  and  remunerative  to  permit 
much  Pontianak  rubber  to  come  to  this  country.  At  the  present 
prevailing  market  prices,  it  seems  obvious  that  the  rubber  portion 
must  be  handled  as  a  by-product  only.2 

1  J.  Ind.  Eng.  Chem.  7,  747-50   (1915). 

2  As  an  indication  of  the  disappearance  of  Pontianak  rubber  from  the  market, 
it  is  only  necessary  to  note  that  according  to  reasonably  reliable  statistics,  only 


THE  COMPOSITION  OF  CRUDE  RUBBER          19 

When  the  resin  content  is  materially  reduced,  Pontianak  rub- 
ber is  very  tacky,  and  plastic,  making  it  difficult  to  store,  as  it 
has  the  tendency  to  flow  together  to  form  one  huge,  unmanageable 
mass. 

Plantation  Rubbers.  The  development  of  the  Hevea  on  the 
plantations  of  the  Far  East,  has  reached  such  proportions  as  to 
make  it  the  dominating  feature  of  the  rubber  market.  Fifteen 
years  ago,  plantation  rubber  was  of  small  commercial  importance, 
very  little  of  it  being  produced.  Today,  the  plantations  furnish 
fully  80%  of  the  world's  supply.  The  rapidity  of  the  growth  is 
well  illustrated  in  the  following  figures,  which  while  they  may  not 
be  absolutely  accurate,  are  sufficiently  so  to  show  the  rapidity  of 
the  growth  of  this  phase  of  the  industry: 

Production  oj  Plantation  Rubber. 

Tons 

1903 25 

1904   50 

1905   150 

1906    500 

1907  1,000 

1908  2,000 

1909  4,000 

1910  8,000 

1911  15,000 

1912  30,000 

1913  50,000 

1914  75,000 

1915   110,000 

1916    160,000 

1917    225,000 

1918   190,000 

1919   360,000 

The  time  has  arrived  when  cultivated  rubber  can  be  produced 
so  cheaply  that  the  poorer  grades  of  wild  rubber  have  been 
forced  out  of  the  market,  and  even  the  better  grades  have  suffered 
severely.  The  analyst  may  therefore  expect  less  and  less  to  be 
confronted  with  samples  for  analysis  which  have  been  made  up 
wholly,  or  in  great  part,  of  wild  rubbers.  Only  in  the  Para  grades 
does  there  seem  to  be  any  sort  of  adherence  to  the  old  grades  of 
wild  rubber.  There  are  still  some  specifications  for  various  ma- 
terials, which  insist  upon  the  use  of  Fine  Para  rubber  (although 
unless  some  representative  of  the  purchaser  actually  sees  the 

1000  tons  were  imported  during  1921.  During  the  same  period,  crude  rubber 
imports  were  estimated  to  be  between  275,000  and  300,000  tons.  In  1905-6,  the 
ratio  of  imports  was  2  tons  of  crude  rubber  to  one  ton  of  Pontianak. 


20  THE  ANALYSIS  OF  RUBBER 

material  made,  how  they  arc  going  to  distinguish  good  smoked 
sheets  from  Fine  Para  is  more  than  one  can  say),  and  they  are 
unwilling  to  change  over  to  plantations  because  they  do  not  know 
what  the  effect  of  such  a  change  would  make  on  the  life  of  the 
articles.  Some  rubber  specialties  have  been  made  from  the  same 
formulas,  calling  for  Para  grades,  for  a  number  of  years,  and  still 
continue  to  be  made  in  this  fashion,  although  at  times  it  is 
difficult  to  get  just  the  grades  of  wild  rubber  needed. 

Smoked  SJtcct.  Although  at  times  it  does  not  command  the 
highest  price,  it  is  the  standard  grade  of  plantation  rubber.3  The 
rubber  should  be  clean,  dry,  firm,  of  a  good  color  and  free  from 
more  than  traces  of  mold  or  rust.  The  moisture  content  will  vary 
between  0.3  Vc  and  1.0%.  The  acetone  extract  will  usually  be 
between  2.5 %  and  3.0%,  and  almost  always  will  be  below  4%. 
The  ash  should  be  negligible. 

Pale  Crepe.  Pale  crepe  is  frequently  called  first  latex,  al- 
though the  same  latex  may,  at  the  choice  of  the  plantations,  be 
made  into  either  ribbed  smoked  sheet,  or  pale  crepe.  The  latter 
is  usually  cleaner  than  smoked  sheet;  chemically,  they  are  very 
much  alike.  The  moisture  content  will  average  lower  than 
smoked  sheet,  the  ash  is  negligible,  and  the  resin  content  between 
'2.~y'/o  and  4.0%. 

Smoked  Crepe.  Smoked  crepe  is  usually  cleaner  than  smoked 
sheet  (the  latter  frequently  contains  bark,  etc.),  with  a  lower 
moisture  content,  approaching  that  of  pale  crepe.  The  resins 
seem  to  run  about  the  same;  if  anything,  a  bit  higher  than  the 
average  of  smoked  sheets.  No  other  particular  differences  have 
been  noted. 

Amber  Crepe.  Amber  crepe  comes  in  several  grades,  according 
to  color.  There  is  no  sharp  dividing  line  between  these  grades 
and  the  pale  crepe,  or  even  amongst  themselves.  Some  of  the 
lighter  amber  crepes  are  very  much  like  the  poorer  lots  of  pale 
crepe.  The  resins,  moisture,  and  ash  in  the  paler  colored  amber 
crepes  is  about  the  same  as  for  pale  crepe  or  smoked  sheet;  the 
lower  grades  are  apt  to  be  sticky,  run  high  in  dirt  and  moisture, 
and  by  reason  of  surface  oxidation,  they  may  be  tacky  and  show 
a  higher  acetone  soluble  figure. 

Roll  Brown  Crepe.     Roll  brown  crepe  comes  into  the  market 

3  For  tin;  methods  of  preparation  of  Smoked  Shoot,  and  Crepe,  cf.  Whithy, 
"Plantation  Knhher,  and  tin-  Testing  of  Kuhher." 


THE  COMPOSITION  OF  CRUDE  RUBBER         21 

in  the  form  of  sheets  of  crepe  which  have  been  rolled  up  into 
small  bundles  about  5  to  6  inches  in  diameter,  and  about  10  to  15 
inches  in  length.  It  is  the  lowest  grade  of  plantation  rubber  on 
the  market,  is  very  tacky,  and  dirty,  and  must  always  be  washed 
in  the  factory  before  it  can  be  used.  When  washed  clean,  and 
dried,  it  replaces  acceptably  the  wild  rubbers  which  have  been 
used  in  friction  stocks,  such  as  Guayule,  etc. 

Constituents  of  Crude  Rubber,  Other  Than  the 
Rubber  Hydrocarbons. 

We  have  already  drawn  attention  to  that  portion  of  the  crude 
rubber  which  is  soluble  in  acetone,  and  which  is  known  com- 
mercially as  rubber  resins.  Apart  from  the  dirt,  bark,  and  water, 
which  may  be  included  in  crude  rubber,  but  which  we  cannot 
consider  as  anything  but  contamination,  there  are  some  other  sub- 
stances, which  are  not  rubber,  but  are  nevertheless  found  in  all 
crude  rubbers. 

Resins.  Hevea  rubber  contains,  in  addition  to  the  rubber 
hydrocarbons  from  2%  to  4%  of  resins.  These  resins  are  about 
80%  saponifiable,  and  20%  unsaponifiable.  They  are  soluble  in 
acetone,  alcohol,  chloroform,  and  many  other  organic  solvents. 
The  solution  is  usually  a  pale  yellow  color,  and  the  residue,  when 
the  solvent  has  been  driven  off,  is  light  colored  with  the  consist- 
ency of  butter.  In  the  unsaponifiable  portion,  Whitby*  has 
identified  some  five  substances  from  the  unsaponifiable  portion, 
some  of  which  show  optical  activity,  and  some  give  sterol  reac- 
tions. The  acetone  extract  of  Hevea  rubber  may  go  higher  than 
4%,  but  this  does  not  necessarily  mean  that  the  resin  content  is 
high,  but  rather  that  there  has  been  oxidation  and  depolymeriza- 
tion  of  the  rubber,  producing  by-products  which  also  are  soluble 
in  acetone. 

Insoluble  Matter.  If  we  take  a  sheet  of  pale  crepe,  smoked 
sheet,  etc.,  and  dissolve  it  in  gasoline,  being  careful  not  to  shake 
too  much,  we  will  find  flakes  of  the  crude  rubber  which  will  not 
dissolve.  This  is  what  is  known  as  the  "insoluble  matter."  The 
amount  will  vary  with  the  method  of  preparation;  analyses  have 
run  between  2%  and  6%.  Rubber  prepared  by  the  total  evapora- 

*  Paper  read  at  the  Spring  meeting  of  the  American  Chemical  Society  at 
Rochester,  April  1921.  "Contribution  to  the  knowledge  of  the  resins  of  Hevea 
rubber,"  by  G.  Stafford  Whitby  and  J.  Doolid. 


22  THE  ANALYSIS  OF  RUBBER 

tion  of  the  latex  will  have  the  highest  figure,  whereas  the  ordinary 
methods  of  coagulation  with  acetic  acid,  washing,  etc.,  reduce  this 
figure  considerably.  The  insoluble  matter  resembles  the  proteins, 
and,  according  to  Eaton,  its  fermentation  will  permit  the  forma- 
tion of  nitrogenous  decomposition  products  which  act  as  acceler- 
ators of  vulcanization.  Such  reactions  take  place  in  the  so-called 
slab  rubber,  in  which  the  coagulum  is  only  slightly  pressed,  and 
which  retains  a  large  amount  of  the  non-soluble  substances  in 
the  latex. 

While  the  insoluble  matter  may  be  shown  by  treating  the  orig- 
inal sheet  with  gasoline  as  described  above,  it  is  next  to  impossible 
to  wash  out  all  of  the  rubber,  so  that  we  cannot  depend  upon 
this  separation  as  a  means  of  a  quantitative  separation.  The 
nitrogen  factor  is  obtained  by  dividing  the  weight  of  the  nitrogen- 
containing  substance  by  the  nitrogen  it  contains;  one  determines 
the  nitrogen  and  multiplies  by  this  factor  to  arrive  at  the  total 
amount  of  nitrogen-substance  present.  This  factor  varies  con- 
siderably, but  6.25  is  a  fair  average,  and  will  give  results  near 
enough  to  the  truth  to  be  acceptable  for  all  practical  purposes. 
In  the  determination  of  glue  by  the  Kjeldahl  method,  this  in- 
soluble matter  appears  as  a  conflicting  element  in  the  determina- 
tion, and  must  be  taken  into  account. 

The  best  rubbers  are  clean  and  dry,  and  have  practically  no 
ash.  A  high  ash  indicates  a  rubber  which  has  been  poorly 
washed  or  which  has  since  picked  up  dirt,  sand,  etc. 

There  are  usually  small  amounts  of  substances,  whose  composi- 
tion we  do  not  know,  but  which  we  recognize  by  the  fact  that  they 
act  as  accelerator  of  the  vulcanization  process.  In  amount,  they 
are  negligible,  except  in  the  case  of  compounds  composed  entirely 
of  rubber  and  sulfur,  when  their  presence  or  absence  may  bear  an 
important  part  in  securing  the  proper  degree  of  vulcanization. 

Tests  for  Crude  Rubber. 

Crude  rubber  may  contain  dirt,  bark,  moisture,  resins,  proteins, 
and  oxidized  or  depolymerized  rubber.  Bark,  dirt,  moisture,  arid 
any  water-soluble  substances,  are  grouped  together  as  "loss  on 
washing." 

Loss  on  Washing.  For  plantation  rubbers,  in  which  the  mois- 
ture and  dirt  is  usually  very  low,  a  5  Ib.  to  10  Ib.  sample  will  suf- 


THE  COMPOSITION  OF  CRUDE  RUBBER          23 

fice.  The  sample  should  be  taken  in  small  pieces  from  different 
parts  of  the  lot,  and  at  least  every  five  cases  should  be  sampled. 
If  the  sample  thus  taken  proves  to  be  too  large  to  handle,  it  can 
be  weighed,  broken  down  on  the  mill,  and  a  smaller  sample  taken 
from  this  broken  down  rubber.  The  latter  should  be  weighed 
when  cool,  in  order  to  ascertain  whether  or  not  any  loss  in  weight 
has  taken  place.  For  wild  rubbers,  not  less  than  50  Ibs.,  and  pref- 
erably 100  Ibs.  should  be  taken  for  the  loss  in  washing  test; 
afterwards,  for  the  other  determinations,  a  smaller  sample  may 
be  drawn  from  the  washed  and  dried  rubber.  Even  greater  care 
must  be  exercised  in  sampling  wild  rubber,  because  of  the  uneven- 
ness  in  size,  cleanliness,  moisture,  etc.,  of  the  various  balls  or  lots 
of  wild  rubber.  Fine  Para,  for  example,  may  be  sampled  by  cut- 
ting the  balls  into  quarters,  until  about  50  Ibs.  are  obtained. 
Dirtier  rubbers,  or  those  which  will  vary  more  from  lot  to  lot, 
should  be  sampled  up  to  100  Ibs.  In  a  later  chapter,  we  propose 
to  deal  more  at  length  with  this  subject  of  sampling,  but  suffice 
it  to  say  here  that  unless  the  proper  care  is  exerted  to  make  the 
sample  drawn  for  this  test  one  which  is  of  the  same  average 
quality  as  the  lot,  the  entire  work  of  testing  is  worse  than  if  it 
were  not  done  at  all,  for  it  may  lead  to  totally  false  results.  The 
rubber  should  be  washed  immediately  after  the  sample  has  been 
drawn  and  weighed. 

Plantation  rubber  may  be  washed  directly,  without  any  pre- 
vious treatment;  wild  rubbers  should  be  heated  in  hot  water  to 
soften  them,  and  render  them  more  plastic,  so  as  to  facilitate  the 
operation.  The  rubber  is  washed  in  the  usual  factory  manner, 
and  then  dried  in  a  vacuum  dryer.  After  removal  from  the 
vacuum  dryer,  the  rubber  is  cooled,  and  weighed,  and  the  loss 
noted. 

A  new  sample  of  about  1000  grams  is  taken  from  different  parts 
of  the  washed  and  dried  sample,  and  united  by  passing  several 
times  through  a  laboratory  mill.  Five  grams  are  weighed  out, 
sheeted  thin  on  the  laboratory  mill  (care  must  be  taken  to  see 
that  no  mechanical  loss  occurs),  and  dried  to  constant  weight  at 
100C.  A  laboratory  vacuum  oven  may  be  used,  but  the  tempera- 
ture should  be  less  than  100C,  since  with  the  reduced  pressure 
the  higher  temperature  is  not  necessary,  and  there  is  less  likeli- 
hood of  damage  to  the  rubber  at  the  lower  temperature.  The  loss 
on  drying  the  5  gr.  sample,  plus  the  shrinkage  during  washing, 


24  THE  ANALYSIS  OF  RUBBER 

give*  the  total  loss  in  weight,  and  should  be  calculated  to  per- 
centage, based  upon  the  original  weight  of  the  sample. 

Itt  sins,  ^lieet  out  thin,  5  gr.  of  rubber,5  calculated  to  the 
dry  basis,  and  wrap  in  filter  paper  which  has  previously  been 
extracted  with  acetone,  place  in  the  extraction  flask,6  and  extract 
continuously  with  acetone  for  eight  hours.  Remove  the  solvent, 
dry  the  ilask  and  contents  to  constant  weight  at  90C  and  calcu- 
late to  percentage.  The  color,  hardness,  and  odor  of  the  extract 
should  be  noted. 

Moisture.  It  is  sometimes  desirable  to  know  simply  the  mois- 
ture in  the  original  sample.  This  is  not  practicable  with  most 
wild  rubbers,  where  the  moisture  is  very  unevenly  distributed, 
but  with  plantation  rubbers  it  is  quite  feasible,  and  often  a 
valuable  figure. 

Cut  up  5  grams  into  small  pieces,  dry  to  constant  weight  in  an 
inert  atmosphere  at  90C.  Calculate  to  percentage. 

Nitrogen.  A  1  gr.  sample  is  placed  in  a  Kjeldahl  flask,  with 
10  gr.  of  potassium  sulfate,  50  cc.  of  cone,  sulfuric  acid  and  1  gr. 
of  copper  sulfate.  Heat  for  three  to  four  hours  (it  is  not  neces- 
sary for  the  solution  to  become  clear),  transfer  to  a  distilling 
flask,  make  the  solution  alkaline  with  caustic  soda,  and  distil  the 
ammonia  into  standard  sulfuric  or  hydrochloric  acid.  Titrate  the 
excess  of  acid  with  standard  sodium  carbonate,  using  methyl 
orange  or  methyl  red  as  indicator. 

Various  determinations  on  the  amount  of  nitrogen  in  the  in- 
soluble matter,  have  given  figures  ranging  between  12%  to  16%.7 
The*  usual  factor  of  6.25  will  give  a  conservative  figure  for  the 
proteins,  but  it  is  likely  that  8.0  or  even  higher,  may  frequently 
be  the  more  correct  value.  It  will  be  seen  from  these  figures  that 
the  determination  of  nitrogen  does  not  signify  very  much. 

Curing  Tests.  It  is  desirable  not  merely  to  know  the  chemical 
composition  and  the  loss  on  washing  of  crude  rubber,  but  also  to 
know  something  of  its  vulcanizing  properties.  For  this  purpose, 
a  standard  formula  should  be  employed,  a  series  of  cures  made 
from  this  mix,  and  stress-strain  curves  drawn  for  each  cure. 

8  It  is  conv<'ui<-nt,  if  not  pressed  for  time,  to  take  the  dried  rubber  from  the 
moisture  determination  in  loss  on  "washing.  This  simplifies  the  correction,  but 
in  HO  doing,  it  must  be  seen  that  the  sample  has  not  been  altered  during  the 
drying,  by  oxidation,  or  depolymerlzation. 

8  Cf.   Acetone   extraction,   under   methods   of   analysis,   p:ige   (JS. 

'Cf.  Schmitz,  Gumrni  Ztg.  27,  lOSf,.  ll.'il;  Spence  and  Kratz,  Koll.  Zeit.  H, 
202-77  (1014). 


THE  COMPOSITION  OF  CRUDE  RUBBER          25 

The  question  of  a  standard  formula  is  one  which  may  not  be 
dismissed  lightly.  At  present,  many  of  the  plantation  and  factory 
chemists  are  using  a  mixture  of  rubber  and  sulfur.  This,  how- 
ever, is  open  to  serious  objection,8  and  a  less  objectionable  pro- 
cedure, even  granting  that  the  formula  itself  may  not  be  the 
best  one,  or  most  suited  for  all  work,  is  to  use  a  formula  contain- 
ing a  small  amount  of  zinc  oxide,  and  sufficient  accelerator  and 
sulfur  to  produce  satisfactory  cures.  One  such  formula  would 
be:  hexamethylenetetramine  0.5% ,  sulfur  4.5%,  zinc  oxide  5%, 
rubber  90%.  This  mixture  contains  enough  sulfur  for  a 
coefficient  of  5.0,9  which  is  higher  than  one  would  ordinarily  go, 
and  zinc  oxide  in  excess  of  that  required  to  neutralize  any  organic 
acids  in  the  rubber,  and  provide  a  basic  mix  for  vulcanization, 
since  practically  all  organic  accelerators  seem  to  work  better 
under  such  conditions.10  Particular  pains  should  be  taken  regard- 
ing the  quality  of  the  zinc  oxide,  sulfur,  and  accelerator;  they 
should  be  of  C.  P.  grade,  and  not  just  the  commercial  stuff  used 
in  the  factory.  Such  grades  are  to  be  found  in  the  market,  and 
are  worth  the  extra  cost.  It  is  not  without  the  bounds  of  reason 
that  much  of  our  unexplainable  vagaries  in  rubber  testing  is 
really  traceable  to  the  impurities  in  the  pigments,  and  not  to  the 
rubber  itself. 

Needless  to  say,  perhaps,  the  results  depend  largely  on  the 
cleanliness  and  technique  in  mixing  and  curing,  the  accuracy  of 
the  thermometers,  and  the  accuracy  of  the  testing  machines.  No 
tests  should  be  made  until  at  least  48  hours  after  vulcanization.11 

•  Cf.  J.  B.  Tuttle,  Variability  of  Crude  Rubber,  J.  Ind.  Eng.  Chem.  13f  519-22 
(1921). 

•The  sulfur  coefficient,  sometimes  called  the  coefficient  of  vulcanization,  is 
the  ratio  of  combined  sulfur  to  the  rubber.  It  is  calculated  by  dividing  the 
percentage  of  rubber  by  the  percentage  of  combined  sulfur. 

It  may  be  mentioned  that  the  coefficient  of  vulcanization  necessary  to  pro- 
duce identical  physical  properties  in  two  or  more  compounds,  is  not  a  constant, 
but  varies  with  the  amount  and  nature  of  the  accelerator  employed,  and  to  a 
lesser  extent  on  the  other  constituents  of  the  compound. 

10  The  real  purpose  of  the  use  of  the  added  organic  accelerator  and  the  zinc 
oxide  should  not  be  lost  sight  of  in  any  discussion  of  the  advisability  of  using 
this  or   any  similar  formula.     It   has   been   shown   that   crude  rubber   contains 
varying  amounts  of  natural  organic  accelerators,   and  we  must  eliminate  their 
effect  if  we  are  to  study  the  actual  variation  of  the  rubber  itself. 

11  There  are  some  who  believe  that  24  hours  is  sufficient  to  permit  the  rubber, 
samples  to  reach  equilibrium.     At  times,  we  have  taken  samples  from  the  vul- 
canizing  press,  and  after  cooling   in   running  water,   tested   them   immediately. 
But  where  results  of  today  are  to  be  compared  with  those  of  the  past,  or  with 
those  to  be  obtained  in  the  future,  the  only  safe  procedure  is  to  allow  the  full 
48  hours,  so  that  such  comparisons  as  may  be  made  will  be  made  under  identical 
circumstances,  and  any  differences  noted  will  be  real  ones,  and  not  those  caused 
by  the  fact  that  at  times  samples  had  not  yet  reached  equilibrium. 


Chapter  III. 
The  Preparation  of  Eubber  Compounds. 

The  art  and  science  of  preparing  rubber  compounds  is  some- 
thing which  may  well  deserve  treatment  of  its  own.  It  is  not 
the  intention  to  explore  the  whys  and  wherefores  of  the  matter, 
for  many  of  the  commercial  compounds  just  "grew  up"  as  time 
went  on,  a  little  of  a  new  material  here,  and  a  little  less  of  an  old 
one  there,  until  at  present  they  are  so  complicated  that  even  the 
owners  of  the  formulas  are  afraid  to  make  any  further  alterations. 
On  the  other  hand,  we  have  a  very  large  number  of  formulas 
which  have  been  constructed  on  the  basis  of  the  definite  physical 
and  chemical  properties  of  such  a  mixture  as  determined  by 
years  of  research.  Irrespective  of  why  it  was  used,  the  analyst  is 
primarily  interested  only  in  what  materials  are  likely  to  be 
used.1  Moreover,  it  is  utterly  impossible  to  include  every  article 
which  has  ever  been  used  in  rubber  manufacture,  but  only  those 
which  have  really  attained  some  commercial  importance,  and 
hence  are  likely  to  be  encountered  in  an  analysis. 

Crude  Rubbers.  In  the  preceding  chapters,  the  general  proper- 
ties of  the  most  important  crude  rubbers  were  given.  This  is 
probably  as  good  a  time  as  any  to  draw  attention  to  the  fact  that 
seldom  will  one  find  a  single  kind  of  crude  rubber  in  a  rubber 
compound.  Coarse  Para  will  be  mixed  with  Fine  Para,  or  amber 
crepes  will  be  mixed  with  smoked  sheets  or  pale  crepe.  It  may 

aAt  the  time  of  writing,  the  situation  with  respect  to  crude  rubber  is  such 
that  the  preparation  of  a  new  compound  is  a  more  than  usually  serious  prob- 
lem. With  the  best  grades  of  plantation  rubber  selling  around  15  cents  a  pound, 
the  saving  In  the  use  of  reclaimed  rubbers  and  substitutes  is  questionable,  if 
we  consider  that  such  materials  are  to  replace  the  rubber.  Some  reclaimed 
rubbers  may  have  an  added  value  on  account  of  the  active  fillers,  such  as  zinc 
oxide  and  gas  black  or  lamp  black,  which  they  may  contain  ;  or  we  may  use 
reclaims  and  substitutes  in  special  cases  on  account  of  special  properties  which 
they  Impart  However,  it  is  incredible  that  such  conditions  as  now  prevail  are 
to  continue  indefinitely,  and  hence  we  are  proceeding  on  the  basis  that  the 
normal  price  for  crude  rubber  will  be  from  25  to  30  cents  (if  not  higher),  and 
at  this  price  the  use  of  certain  grades  of  reclaims  and  substitutes  will  effect 
savings  in  costs,  and  hence  the  analyst  may  expect  to  find  them  in  examining 
manufactured  articles. 

26 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      27 

seem  superfluous,  but  it  is  safer  to  call  attention  to  the  fact  that 
replacing  Fine  Para  with  Coarse  Para,  or  smoked  sheet  with  am- 
ber crepe,  is  merely  a  matter  of  economy ;  the  rubbers  used  are  not 
as  good  as  those  they  replace,  and  the  quality  of  the  compound  is 
lowered.  It  is  purely  a  question  of  deciding  whether  or  not  the 
properties  of  the  compound  are  sufficient  to  meet  the  demands 
of  the  service.  On  the  other  hand,  rubber  such  as  Pontianak, 
Guayule,  roll  brown  crepe,  etc.,  when  used  as  softeners,  are  used 
independently  of  their  cost,  and  their  use  has  continued  in  many 
cases  when  they  cost  practically  as  much,  or  even  more  than  the 
so-called  better  rubbers.  These  points  are  worth  bearing  in  mind 
in  figuring  out  the  probable  formula  from  the  analysis  of  a 
rubber  compound. 

Reclaimed  Rubber.  We  have  seen  that  the  rubber  hydrocarbon 
can  combine  with  sulfur  until  the  compound  (C10H16  S2)x  is 
reached,  when  the  ratio  of  rubber  to  sulfur  is  136:64.  In  the 
ordinary  soft  vulcanized  articles,  the  sulfur  coefficient  is  between 
1.5  and  5.0,  depending  upon  the  type  of  accelerator,  and  the 
degree  of  vulcanization.  Such  material  is  able  to  take  up  fur- 
ther quantities  of  sulfur  to  form  a  new  compound  with  a  higher 
coefficient,  which,  while  somewhat  harder  than  the  material  from 
which  it  was  made,  may  still  be  of  service.  Each  addition  of 
sulfur,  other  conditions  being  equal,  produces  a  harder  product 
than  before,  until,  with  the  maximum  amount  of  sulfur  which 
may  be  added,  we  reach  the  product  ebonite.  The  hardness  of 
the  rubber  itself  is  frequently  lessened  by  the  admixture  of  soft- 
ening oils,  and  the  partial  depolymerization  which  takes  place 
produces  a  soft  and  tacky  substance,  which  also  helps  to  counter- 
act the  hardening  effect  of  the  additional  sulfur. 

Before  vulcanized  rubber  can  be  used  a  second  time,  it  must 
be  put  into  condition  to  be  mixed  in  a  homogeneous  manner  with 
new  rubber.  There  are  two  general  processes  employed,  (a)  the 
acid  reclaiming  process;  and  (6)  the  alkali  reclaiming  process. 
These  processes  serve  to  remove  any  fabric  which  may  be  present, 
the  free  sulfur,  and,  of  course,  some  of  the  fillers,  both  organic 
and  inorganic.  In  the  latter  case,  the  amount  and  nature  of  the 
fillers  removed  will  depend  largely  upon  the  process  which  is  used 
and  the  chemical  nature  of  the  fillers.  Zinc  oxide  and  whiting 
are  largely  removed  in  the  acid  process,  zinc  oxide  to  a  slight 
extent  in  the  alkali  process,  while  gas  black  and  lamp  black  are 


28  THE  ANALYSIS  OF  RUBBER 

unaffected  by  either.  Oil  substitutes  are  not  attacked  in  the 
acid  process,  but  are  almost  completely  removed  by  the  alkali 
process. 

These  processes  of  reclaiming  do  not  reverse  the  vulcanization 
process;  on  the  contrary,  if  there  be  any  quantity  of  free  sulfur 
present,  part  of  it  will  combine  with  the  rubber  during  the  re- 
claiming, the  sulfur  coefficient  being  higher  afterward  than  before. 
Other  processes  have  been  worked  out  for  the  purpose  of  taking 
out  the  sulfur  and  restoring  the  double  bond,  in  which  case  we 
would  expect  a  product  similar  to  new  rubber,  and  which  would 
vulcanize  in  the  same  manner.  This  is  the  ideal  towards  which 
the  researches  have  been  directed,  but  it  must  be  admitted  that 
as  yet  we  have  fallen  far  short  of  the  ideal,  and  the  reclaimed 
rubber  encountered  in  vulcanized  compounds  has  been  made  by 
one  or  the  other  of  the  two  methods  mentioned  above,  or  some 
variation  of  them. 

Reclaimed  rubber  is  added,  under  normal  market  conditions, 
first  of  all  because  it  is  cheaper.  Certain  grades  may  be  used 
because  they  give  desirable  properties;  for  example,  vulcanized 
reclaimed  rubber  resists  oil  better  than  does  new  rubber,  and  the 
use  to  which  the  article  is  to  be  put  is  worthy  of  notice  in  deciding 
whether  or  not  reclaimed  rubber  has  been  used  on  account  of  its 
cost,  or  because  in  the  case  in  question,  it  is  actually  better. 

•  In  the  manufacture  of  pneumatic  tires,  there  is  always  a  con- 
siderable amount  of  fabric  trimmings,  containing  a  large  amount 
of  new,  unvulcanized  rubber.  By  the  acid  reclaiming  process,  the 
fabric  may  be  entirely  removed,  with  a  considerable  portion  of 
the  sulfur,  without  appreciably  causing  the  rubber  and  sulfur  to 
combine.  The  product,  known  as  "reclaimed  or  pure  gum  fric- 
tion," is  a  valuable  adjunct  in  rubber  compounding. 

Oil  Substitutes. 

In  the  preparation  of  certain  articles,  where  the  highest  physi- 
cal properties  were  not  of  primary  importance,  substitutes  for 
rubber  have  been  used  in  order  to  lessen  the  cost  of  manufacture 
(cf.  footnote,  page  26).  One  group  of  such  substitutes  is  made 
from  oils  of  various  kinds,  and  these  substitutes  are  known  com- 
mercially as  "oil  substitutes." 

When  drying,  or  semi-drying  oils,  such  as  linseed,  soya,  corn, 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      29 

cottonseed,  and  similar  oils,  are  treated  with  sulfur  or  sulfur 
chloride,  a  solid  plastic  mass  is  obtained.  These  products  have 
been  called  vulcanized  oils,  because  of  the  similarity  of  the 
processes  of  preparation  with  those  of  rubber.  The  reaction  with 
sulfur  requires  heating,  and  the  product  varies  in  color  from  a 
light  to  a  very  dark  brown,  or  even  black.  The  sulfur  chloride 
combines  at  ordinary  temperatures,  giving  us  the  so-called  "white 
substitutes." 

Mixed  with  these  substitutes  are  various  gums  and  oils,  pro- 
ducing an  almost  endless  number  of  combinations.  This  need  not 
bother  the  analyst,  however,  for  the  treated  oils  are  insoluble  in 
acetone  and  chloroform,  whereas  the  untreated  oils  and  gums  are 
usually  soluble  in  one  or  the  other  of  these  solvents.  They  may 
also  be  loaded  with  mineral  pigments  of  various  kinds. 

Tests  of  Oil  Substitutes. 

An  examination  of  the  raw  material  should  cover  the  un- 
changed oil,  loss  on  heating  at  100C,  free  sulfur,  and  ash.  Un- 
changed oil  acts  in  a  totally  different  manner  from  the  true 
substitute,  and  the  free  sulfur  is  especially  important,  since  it  is 
capable  of  combining  with  the  rubber  during  vulcanization; 
hence  any  free  sulfur  present  must  be  taken  into  account  when 
figuring  the  amount  of  sulfur  to  be  added  as  such  to  the  rubber 
compound. 

Unchanged  Oil  Reduce  the  sample  to  a  fine  state  of  division 
by  crumbling  or  cutting.  Extract  2  gr.  with  acetone  for  eight 
hours;  dry  the  extract  to  constant  weight  at  90C,  cool  and  weigh. 

Free  Sulfur.  Treat  the  dried  acetone  extract  with  50  to  75  cc. 
of  water,  and  2  to  3  cc.  of  bromine,  heat  until  colorless,  or  nearly 
so,  filter  through  a  folded  filter;  heat  the  filtrate  to  boiling,  add 
10  cc.  10%  barium  chloride,  and  determine  the  precipitated 
barium  sulfate  as  usual.  Calculate  to  sulfur,  and  deduct  the 
percentage  of  free  sulfur  from  the  total  acetone  extract.  The 
remainder  is  the  unchanged  oil. 

Loss  in  Weight.  Dry  a  2  gr.  sample  in  a  neutral  atmosphere  at 
90/1000  until  constant  weight  is  secured. 

Mineral  Fillers.  Ignite  a  1  gr.  sample,  cool  the  residue  and 
weigh.  Pure  oil  substitutes  should  have  practically  no  ash;  if 
any  pigments  are  added,  the  amount  will  be  such  as  to  leave  no 


30  THE  ANALYSIS  OF  RUBBER 

doubt  in  the  analyst's  mind  as  to  whether  such  additional  was 
accidental,  or  not.  Oil  substitutes  are  usually  found  in  amounts 
of  from  1%  to  5%,  although  we  have  seen  some  German  made 
rubber  tubing  that  had  nearly  50%  of  oil  substitute. 

Mineral  Rubber. 

The  mineral  rubbers  are  asphaltic  or  bituminous  hydrocarbons 
.obtained  either  from  natural  or  artificial  sources.  The  natural 
sources  are  from  the  minerals  gilsonite  and  elaterite,  while  the 
artificial  mineral  rubbers  are  made  largely  from  the  blown  oils 
from  petroleum  residues.2 

Mineral  rubber  possesses  a  melting  point  above  that  of  the 
usual  vulcanization  range,  but  its  plasticity  enables  it  to  be 
worked  readily  at  much  lower  temperatures.  In  amounts  up  to 
7  volumes,3  it  materially  improves  the  tensile  properties.  It 
serves  to  soften  the  uncured  stock,  makes  it  tackier  reduces 
blooming,  and  in  a  variety  of  ways  proves  itself  to  be  an  asset  to 
a  rubber  compound.  It  improves  the  waterproofing  properties 
of  rubber. 

Owing  largely  to  the  differences  in  the  source  of  supply,  and  to 
the  various  methods  of  preparation,  the  chemical  and  physical 
properties  vary  widely.  The  acetone-soluble  matter  varies  enor- 
mously, running  as  low  as  17%,  and  as  high  as  60%,  the  higher 
percentages  being  the  more  common  occurrence.  Chloroform  will 
dissolve  part  of  the  residue,  equal  to  about  10%  of  the  whole. 
They  may  contain  as  much  as  10%  of  their  weight  in  sulfur,  all  of 
which  is  chemically  combined.  There  is  always  a  fair  sized 
amount  which  is  soluble  neither  in  acetone  nor  chloroform. 

While  the  solvents  do  not  give  us  exact  data  as  to  the  quantita- 
tive figures  on  mineral  rubber,  the  color  of  the  chloroform  extract 
is  a  very  reliable  index  in  determining  the  presence  or  absence  of 
this  material.  When  present,  this  extract  is  deep  brown  to  black 

•For  the  best  and  most  recent  work  on  Mineral  Rubber,  consult  the  article 
by  C.  Olin  North,  "Mineral  Rubber,"  read  at  the  meeting  of  the  Rubber  Division 
of  the  American  Chemical  Society  at  New  York,  September  6th  to  10th,  1921. 
Abstracts  of  this  paper  are  to  be  found  in  the  "India  Rubber  World."  65,  191-2 
(1921),  and  "The  Rubber  Age,"  10,  130-1  (1921). 

*  Since  the  specific  gravity  of  the  materials  used  in  rubber  compounding  varies 
widely,  it  affords  a  more  exact  method  of  comparing  the  effect  of  the  different 
substances  if  they  are  compared  on  the  basis  of  volume  rather  than  weight. 
The  volume  is  referred  to  the  total  volume  of  rubber,  the  latter  being  taken 
as  100, 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      31 

in  color,  and  is  not  likely  to  be  confused  with  any  other  class  of 
material  used  in  rubber  manufacture. 

During  vulcanization,  the  percentage  of  soluble  matter  may 
change  somewhat;  the  acetone  extract  is  usually  somewhat  lower 
than  when  the  material  itself  is  subjected  to  extraction.  The 
chloroform  extract  shows  little  change.  Various  explanations 
have  been  offered:  (1)  that  the  mineral  rubber  unites  with  the 
rubber;  (2)  it  combines  with  the  sulfur  to  form  insoluble  prod- 
ucts; (3)  the  dispersion  of  the  mineral  rubber  on  the  crude  rubber 
produces  an  adsorption  effect,  and  renders  the  former  more  diffi- 
sult  to  dissolve  out  of  the  mix.  Of  these,  the  second  seems  to  be 
the  most  plausible,  although  admittedly  the  other  two  are 
possibilities. 

Mineral  rubber  has  a  specific  gravity  of  about  1.00;  the  hard- 
ness varies  according  to  the  melting  point.  The  melting  point 
is  anything  that  may  be  desired,  but  the  most  popular  grade  is 
that  melting  in  the  neighborhood  of  310F. 

North  *  has  determined  that  the  best  results  with  mineral  rub- 
ber are  obtained  when  the  proportion  is  7  volumes  of  mineral 
rubber  to  100  of  rubber.  One  is  more  likely  to  meet  with  less 
rather  than  with  more  than  this  amount. 

Tests  for  Mineral  Rubber. 

Acetone  Soluble.  Extract  with  acetone  for  four  hours,  a  1  gr. 
sample  of  the  mineral  rubber;  dry  to  constant  weight,  at  100C. 

Chloroform  Extract.  Without  drying  the  sample  which  has 
been  extracted  with  acetone,  extract  with  chloroform  for  two 
hours,  or  longer  if  at  the  end  of  that  period  the  solvent  is  still 
colored.  Dry  the  extract  to  constant  weight,  at  100C. 

Ash.  Ignite  1  gr.  in  a  porcelain  crucible,  cool  and  weigh.  The 
residue  should  be  negligible. 

Insoluble  Matter.  The  difference  between  100%  and  the  sum 
of  the  acetone  and  chloroform  extracts,  and  the  ash,  shall  be 
called  "insoluble  matter." 

Mineral  Hydrocarbons. 

The  mineral  hydrocarbons  may  be  divided  into  two  classes, 
hard  and  soft.  The  former  include  ozokerite,  ceresin,  and  par- 
affin; the  latter,  petrolatum  and  heavy  mineral  oil.  The  hard 

*Loc.  clt. 


32          '  THE  ANALYSIS  OF  RUBBER 

hydrocarbons  are  useful  for  their  waterproofing  effect,  and  are 
to  be  found  largely  in  materials  intended  for  electrical  purposes, 
such  as  insulated  wire  and  cable.  The  soft  hydrocarbons  are  used 
purely  as  softeners,  to  facilitate  the  handling  of  the  stocks  in  the 
factory,  and  whereas  the  hard  hydrocarbons  are  without  any 
serious  effect  on  the  aging  qualities,  the  soft  hydrocarbons  have  a 
decided  deteriorating  effect,  and  must  be  used  in  small  quantities. 
The  explanation  of  this  effect  would  appear  to  be  that  the  mineral 
oils  are  solvents  for  vulcanized  rubber  (as  previously  stated, 
however,  this  is  not  a  true  solution,  but  rather  a  depolymerization 
preceding  solution) . 

Mineral  hydrocarbons  are  rarely  used  to  a  greater  extent 
than  5rc,  and  in  the  greatest  number  of  cases  the  amount  used 
is  between  1%  and  2%. 

Ozokerite.  Ozokerite  is  a  natural  product,  found  in  Austria, 
Russia  and  southern  Utah.  It  is  dark  brown  to  black  in  color, 
with  a  specific  gravity  of  about  0.90.  The  melting  point  should 
exceed  65C  (150F).  Ccresin  is  ozokerite  which  has  been  purified 
by  treatment  with  fuming  sulfuric  acid;  it  is  pale  yellow  in  color, 
with  a  resinous  luster,  non-crystalline  in  appearance,  but  in  other 
respects,  similar  to  the  parent  substance. 

Paraffin.  Paraffin  is  a  hard,  white,  crystalline  substance,  com- 
posed of  the  higher  boiling  hydrocarbons  from  petroleum.  Its 
specific  gravity  is  about  0.90,  the  melting  point  almost  anything 
that  one  desires,  from  soft  paraffin  which  borders  closely  or 
petrolatum,  to  the  hard  paraffins  with  melting  points  around  60C 
Ozokerite  and  ceresin  are  so  much  higher  in  price  than  paraffin 
that  the  temptation  for  adulteration  is  very  great,  and  this  is  al 
the  more  true  because  of  the  fact  that  paraffin,  which  is  uscc 
largely  as  the  adulterant,  is  so  near  in  chemical  and  physica 
properties  that  rather  large  amounts  can  be  added  without  fear  o 
detection.  Ceresin  in  the  pure  state  is  much  less  crystalline  thai 
paraffin,  and  less  brittle,  but  it  is  doubtful  if  these  advantage: 
warrant  the  extra  cost  of  the  pure  article. 

Paraffin  and  ceresin  have  the  peculiar  property  of  working 
toward  the  surface  of  a  rubber  article,  much  in  the  same  manne: 
as  sulfur  "blooms."  It  appears  within  a  few  days  after  vulcani 
zation,  and  if  a  slab  of  rubber  containing  paraffin  be  left  un 
touched  for  say  six  months,  it  is  possible  to  scrape  a  considerabL 
quantity  of  clean  paraffin  from  the  surface  (possibly  mixed  wit! 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      33 

sulfur  if  the  free  sulfur  is  high).  This  fact  is  important  in 
analyzing  such  materials,  for  the  ordinary  handling,  cleaning, 
etc.,  in  preparing  a  sample  for  analysis,  will  remove  an  appre- 
ciable quantity,  and  hence,  on  this  account,  irrespective  of  the 
errors  of  analysis,  the  determination  of  paraffin  or  ceresin  is 
likely  to  be  low  rather  than  high. 

Oils  and  Waxes. 

Rubber  compounds  may  be  made  suitable  for  calendaring,  tub- 
ing, and  other  operations,  either  by  excessive  working  on  the 
mixing  mills,  or  by  the  use  of  elevated  temperatures.  Both 
methods  are  objectionable  in  one  sense  or  another,  the  excessive 
working  breaks  down  the  rubber,  producing  a  sticky,  porous 
material  which  is  difficult  to  handle,  to  say  nothing  of  its  poorer 
tensile  properties.  High  temperatures  are  to  be  avoided  in  the 
preliminary  stages  of  manufacture,  especially  with  organic  accel- 
erators, since  some  of  the  latter  become  very  active  at  moderately 
low  temperatures,  and  a  partial  vulcanization  will  be  effected 
(what  is  technically  known  as  "burnt"  or  "scorched"  stock). 
One  method  for  avoiding  these  difficulties  is  to  add  a  small 
amount  of  oil  (usually  1%  to  3%),  which  softens  the  rubber 
compound  and  brings  about  satisfactory  working  conditions. 
We  recognize  two  classes  in  these  softeners,  (a)  in  which  the 
oil  or  wax  acts  merely  as  a  softener;  (b)  in  which,  in  addition 
to  its  softening  effect,  it  adds  some  distinct  and  desired  property, 
such  as  tackiness,  etc.  In  class  (a)  we  find  palm  oil,  cottonseed 
oil,  petrolatum  or  vaseline,  and  heavy  mineral  oils;  in  class  (6), 
Burgundy  pitch,  colophony  or  ordinary  rosin,  rosin  oil,  tar  oils, 
etc.  The  former  may  be  expected  in  almost  any  stock,  but  the 
latter  are  used  chiefly  in  cement  stocks,  frictions,  tapes,  etc., 
where  adhesive  properties  have  a  particular  value. 

Palm  Oil  Palm  oil  is  obtained  from  the  fruit  of  the  palm  tree, 
Eloeis  guineensis,  and  the  west  coast  of  Africa  is  practically  the 
only  important  commercial  source  of  this  oil.  Specific  gravity, 
0.921-0.925;  melting  point  27-42C,  solidification  point  37-39C, 
depending  upon  the  age  and  origin  of  the  oil.  Iodine  number, 
53-57;  the  commercial  oil  contains  water,  sometimes  as  much  as 
1% ;  other  impurities  up  to  3%.  It  may  be  adulterated  with  bark 
and  dirt,  and,  before  using,  palm  oil  is  melted,  and  the  clean  oil 


34  THE  ANALYSIS  OF  RUBBER 

skimmed  from  the  surface.  Palm  oil  is  rarely  adulterated  with 
other  oils  or  fats,  hence  it  is  usually  sufficient  to  determine  water, 
total  impurities,  and  the  solidifying  point.  The  color  varies  from 
orange  yellow  to  a  dark,  dirty  red. 

Cottonseed  Oil.  Cottonseed  oil  is  obtained  from  the  seeds  of 
the  cotton  plant,  Gossypium,  of  which  the  principal  species  are 
G.  Herbaceum  in  the  United  States,  and  G.  Barbadense  in  Egypt. 
Choice  crude  oil  should  be  free  from  water  and  foots,  possess  a 
sweet  flavor  and  odor  (i.e.,  should  not  be  rancid) ,  specific  gravity 
0.922-0.925;  solidifying  point  3-4C;  iodine  number  105-110. 


Tests  for  Cottonseed  Oil. 

The  best  known  test  for  cottonseed  oil  is  Halphen's  color  test, 
made  as  follows:  1-3  cc.  of  the  oil  is  dissolved  in  an  equal 
volume  of  amyl  alcohol,  to  this  is  added  1-3  cc.  of  carbon  bisul- 
fide holding  in  solution  \%  of  sulfur.  The  test  tube  is  immersed 
in  boiling  water,  and  the  carbon  bisulfide  driven  off.  A  deep  red 
color  appears  in  about  30  minutes.  The  test  depends  upon  the 
presence  of  some  chromogenetic  substances  which  are  destroyed 
by  high  heating,  so  that  rubber  compounds  containing  cottonseed 
oil  may  not  show  this  test  after  vulcanization. 

Petrolatum.  Petrolatum,  or  vaseline,  may  be  either  light  or 
dark  colored.  Its  specific  gravity  is  between  0.85  and  0.90.  At 
ordinary  teniperatures,  it  is  a  soft  paste,  but  at  40  to  50C  it 
melts  to  a  clear  fluorescent  oil.  It  is  not  altered  in  composition 
during  vulcanization,  and,  unlike  paraffin,  it  remains  distributed 
throughout  the  compound  after  vulcanization,  and  does  not  bloom 
to  the  surface. 

Heavy  Mineral  Oils.  The  heavy  mineral  oils  are  purely  soften- 
ers, but  are  more  likely  to  be  found  as  component  parts  of 
reclaimed  rubber  and  substitutes,  than  actually  added  to  com- 
pounds as  such.  They  act  in  practically  the  same  manner  as 
petrolatum. 

Burgundy  Pitch.  Burgundy  pitch  is  more  important  for  its 
adhesive  properties  than  as  a  softener,  although  it  acts  in  both 
capacities.  It  is  a  dark,  brittle  substance,  with  a  resinous  luster, 
and  a  specific  gravity  of  about  1.10.  It  is  soluble  in  acetone. 
It  is  obtained  from  the  Norway  spruce,  Picea  Excelsa,  by  scarifi- 
cation of  the  trees,  and  collecting  the  resin  after  it  has  hardened. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      35 

The  volatile  oils  which  are  present  in  the  crude  resin  are  removed 
by  boiling  with  water.  It  contains  considerable  bark  and  dirt, 
and  must  be  purified  by  melting  and  filtering  through  sieves. 
It  is  frequently  found  in  low  grade  frictions,  insulating  tape, 
cements,  etc. 

Burgundy  pitch  is  composed  largely  of  abietic  anhydride,  and 
gives  a  positive  reaction  with  the  Liebermann-Storch  test.  It  is 
so  near  ordinary  rosin  in  composition  that  the  latter  is  fre- 
quently used  as  an  adulterant,  and  it  is  one  that  is  exceedingly 
difficult  to  detect. 

Rosin,  or  Colophony.  Rosin  is  the  residue  remaining  in  the  still 
in  the  separation  of  oil  of  turpentine  from  crude  turpentine.  Its 
principal  constituent  is  abietic  anhydride.  Rosin  is  about  90% 
saponifiable,  the  remaining  10%  consisting  of  rosin  oil.  It  melts 
anywhere  from  100  to  140C,  specific  gravity  1.08.  Its  color 
varies  from  water  white,  pale  amber,  to  black,  but  only  the 
lighter  amber  colors  are  used  in  rubber  manufacture.  It  has  very 
little  softening  power,  but  is  exceedingly  tacky,  so  that  it  can  be 
used  only  in  small  amounts  in  cements,  frictions,  and  varnishes. 

Rosin  Oil.  By  the  destructive  distillation  of  rosin,  we  obtain, 
amongst  other  products,  a  reddish  colored  oil,  commonly  called 
rosin  oil.  Its  boiling  point  is  around  360C,  or  over,  specific 
gravity  0.98-1.10;  it  usually  contains  10%  to  20%  of  rosin,  which 
is  saponifiable,  but  the  remaining  80%  to  90%  is  an  unsaponifi- 
able  hydrocarbon.  It  will  be  noticed  that  rosin  always  contains  a 
small  amount  of  rosin  oil,  and  vice  versa,  hence,  both  substances 
give  the  same  positive  reaction  in  the  Liebermann-Storch  test.5 

Rosin  oil  adds  very  little  to  the  tackiness  of  the  rubber,  and  is 
essentially  a  softener.  It  improves  the  waterproofing  qualities 
of  rubber.6  Rosin  oil  is  not  used  very  extensively,  especially  in 

•The  simplest  way  to  make  this  test  is  to  warm  a  few  drops  of  the  oil  in 
1-2  cc.  of  acetic  anhydride,  cool,  and  to  a  few  drops  on  a  porcelain  test  plate, 
add  a  drop  of  sulfuric  acid  of  sp.  g.  about  1.5.  A  reddish  violet  color  indicates 
rosin  or  rosin  oil.  It  is  believed  that  the  unsaponiflable  portion  is  really  respon- 
sible for  the  color,  and  when  examining  for  rosin  or  rosin  oil,  the  test  may  be 
made  much  more  delicate  by  making  it  upon  the  unsaponiflable  portion.  Bur- 
gundy pitch.  Venice  turpentine  and  similar  resins,  give  practically  the  same 
color,  so  that  the  identification  as  rosin  or  rosin  oil  is  not  absolutely  positive. 

8  Rubber  compounds  are  so  frequently  used  for  waterproofing  and  in  such 
articles  as  rubber  tubing,  hot  water  bags,  etc.,  that  one  is  quite  likely  to  over- 
look the  fact  that  rubber  takes  up  a  large  amount  of  water  when  left  in  contact 
with  it  for  any  length  of  time,  and  this  holds  true  even  after  the  rubber  has 
been  vulcanized.  Pure  gum  sheet,  vulcanized,  has  been  found  to  absorb  as 
much  as  20%  of  its  weight  in  water.  C.  O.  Weber,  in  his  book  on  India 


36  THE  ANALYSIS  OF  RUBBER 

high  grade  goods,  since  it  is  a  solvent,  or  rather  a  depolymerizer 
of  rubber.  The  connection  between  the  two  substances,  rosin  oil 
and  rubber,  can  readily  be  seen  in  the  fact  that  crude  turpentine 
is  composed  largely  of  the  terpenes  sylvestrene  and  australene, 
the  composition  of  which  is  C10H16;  which  form  tetrabromides, 
ozonides,  and  polymerize  easily. 

Tar  Oils.  The  tar  oils  are  the  residues  from  the  destructive 
distillation  of  wood  or  coal,  the  coal  tars  being  the  ones  gener- 
ally used.  They  are  of  varying  composition,  and  act  merely  as 
softeners.  As  a  rule,  they  are  soluble  in  acetone  and  alcohol,  and 
have  a  specific  gravity  of  about  1.00.  Their  properties  depend 
largely  upon  the  source  of  the  crude  material,  and  the  degree  of 
rectification. 

Glue.  The  glue  used  in  rubber  compounding  is  the  ordinary 
granulated  bone  glue.  The  moisture  content  varies  between  7% 
and  12%,  and  the  specific  gravity  is  about  1.25.  Just  as  it  comes, 
it  may  be  mixed  directly  with  rubber  on  a  fairly  warm  mill.  It 
is  best  to  have  the  mixture  refined  while  it  is  still  hot  in  order 
to  thoroughly  break  up  any  particles  of  glue.  Several  other 
methods  are  in  vogue;  3  parts  of  glue  are  heated  with  1  part  of 
water  until  a  smooth  mixture  is  obtained,7  then  cooled  until  it 
sets  to  a  firm  jelly.  This  is  mixed  with  rubber,  and  dried  in  a 
vacuum  dryer.  In  other  preparations,  the  glue  is  melted  and 
mixed  with  oils  or  glycerin,  and  then  allowed  to  cool;  or  it  may 
be  dissolved  in  water,  gas  black  or  other  fillers  stirred  in,  the 
solution  concentrated,  and  the  cooled  mass  mixed  with  rubber. 

The  effect  of  glue  on  rubber  is  to  reduce  the  elongation,  and 
increase  the  permanent  set.  In  many  compounds,  it  has  been 
found  to  exert  a  stabilizing  effect  on  the  cure,  flattening  out  the 

Rubber  and  its  Analysis,  p.  14,  says :  "The  water  absorption  of  vulcanized 
rubber  is  extremely  small,  certainly  not  large  enough  to  appreciably  affect  the 
insulation  of  a  rubber  cable  after  5  years'  continuous  immersion."  Weber  did 
not  state  what  kind  of  a  compound  he  had  in  mind  when  he  made  this  state- 
ment, but  we  have  had  experience  with  40%  fine  Para  compounds,  containing 
about  2%  of  paraffin,  which  became  absolutely  waterlogged  after  about  two 
or  three  years  continuous  immersion  in  water,  and  were  utterly  unfit  for  their 
purpose.  To  secure  the  best  waterproofing  properties,  we  resort  to  the  addition 
of  oils,  waxes,  and  pitches.  This  is  particularly  true  in  electrical  supplies. 

7  At  this  stage,  several  possibilities  are  open.  Some  add  formaldehyde  in 
sufficient  quantities  to  produce  an  insoluble  glue.  Others  have  added  glycerin, 
about  5%  of  the  dry  glue,  and  concentrated  the  solution  until  the  moisture  con- 
tent is  from  15%  to  20%.  The  purpose  of  this  is  to  prevent  the  glue,  on 
cooling,  becoming  hard  and  brittle.  This  glue-glycerin-water  combination  mixes 
readily  with  rubber,  and  in  so  doing,  the  moisture  content  is  substantially 
reduced. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      37 

peak  of  the  vulcanization  curves,  and  reducing  the  danger  of 
either  over  or  under  cures.  It  has  a  special  field  in  rubber  tubing 
for  conducting  gasoline,  and  other  organic  solvents,  reducing 
greatly  the  effect  of  such  solvents  on  the  rubber.  Glue  has  a 
slight  accelerating  effect  on  the  vulcanization. 

Other  Organic  Fillers.  A  large  number  of  organic  substances 
are  used  in  special  articles,  by  reason  of  the  real  or  fancied  im- 
provement in  the  quality  or  service,  from  such  addition,  or  for 
reasons  of  economy.  Rubber  soles  may  be  stiffened  with  ground 
cotton  fabric;  shellac,  hard  gums  and  resins  are  used  in  cements 
and  in  waterproofing;  ground  cork  or  leather  in  some  floor  cover- 
ings, etc.,  etc. 

Vulcanizing  Materials. 

Sulfur.  The  sulfur  used  in  rubber  should  be  dry,  and  free  from 
acid,  sand,  or  other  impurities.  Before  using,  it  should  be  care- 
fully sifted  through  a  50  mesh  screen,  excepting,  of  course,  in  low 
grade  compounds,  where  such  refinements  are  of  no  value.  The 
purpose  of  the  sifting  is  to  remove  dirt,  splinters  of  wood,  etc., 
that  may  come  from  the  container,  and  to  remove  agglomerations 
or  lumps  of  sulfur. 

Tests  for  Sulfur. 

Acidity.  Ten  gr.  of  the  sample  is  placed  in  a  flask,  with 
100  cc.  of  distilled  water,  heated  on  the  water  bath  for  15-30 
minutes,  and  any  acidity  titrated  with  N/10  sodium  carbonate, 
using  methyl  orange  as  the  indicator.  A  blank  is  run  on  the 
water  used.  Not  over  2-3  drops  should  be  required  to  make  the 
solution  alkaline. 

Moisture.  Dry  at  85C  for  one  hour  in  a  neutral  gas,  1  gr. 
of  sample,  cool  and  weigh.  The  loss  should  be  negligible. 

Ash.  Ignite  1  gr.  of  sulfur  in  a  porcelain  crucible,  performing 
the  burning  in  a  hood  with  a  strong  draft.  Cool  the  crucible  and 
weigh.  The  ash  should  be  less  than  1  mg. 

Sulfur  Chloride.  The  sulfur  chloride  used  in  rubber  manufac- 
ture is  the  monochloride,  S2C12.  Since,  however,  chlorine  acts  on 
the  monochloride  to  give  the  dichloride,  there  is  usually  some  of 
the  latter  present  in  commercial  sulfur  monochloride.  Pure 
sulfur  monochloride  has  a  specific  gravity  of  1.709,  boils  at  138C, 
fumes  strongly  in  the  air,  is  decomposed  by  water  forming  sulfur 


38  THE  ANALYSIS  OF  RUBBER 

dioxide,  sulfur  and  hydrochloric  acid.  The  sulfur  liberated  by 
the  reaction  with  water  is  readily  dissolved  by  the  sulfur  chloride. 
It  is  usually  a  red  or  a  deep  orange  color.  The  dichloride,  SC12 
has  a  specific  gravity  of  1.62,  boils  at  64C,  and  at  the  boiling 
point  partially  decomposes  into  S2C12  and  C12. 

The  commercial  sulfur  monochloride  usually  has  a  gravity  be- 
tween 1.65  and  1.70,  and  a  boiling  point  between  115C  and  130C. 

Sulfur  monochloride  should  be  stored  in  a  cool,  dry  spot,  in 
clean  earthenware  jugs  with  tight  fitting  earthenware  stoppers. 
It  should  not  be  exposed  to  the  air,  on  account  of  its  affinity 
for  water.8 

Organic  Accelerators. 

The  number  of  organic  substances  which  accelerate  the  vul- 
canization of  rubber  is  so  great  that  we  have  deemed  it  quite 
unnecessary  to  attempt  to  deal  with  those  which  are  only  of 
casual  interest.  Primarily,  we  are  dealing  with  the  analysis  of 
rubber  goods,  and  are  chiefly  interested  in  the  accelerators  which 
are  now  being  used  commercially,  or  which  show  possibilities  of 
becoming  such.  The  most  widely  used  organic  accelerators  today 
are  aniline,  thiocarbanilide,  and  hexamethylenetetramine,  and 
the  analyst  should  look  first  for  these  three  before  proceeding 
further. 

Most  organic  accelerators  are  used  in  small  amounts.  For 
very  fast  curing  purposes,  such  as  tire  repair  stocks,  the  quan- 
tity may  be  as  high  as  5%  or  6% ;  but  for  ordinary  compounds 
the  amount  is  usually  1%  or  less  of  the  amount  of  rubber  pres- 
ent. The  amount  used  depends  largely  upon  the  time  of  cure 
desired,  and  the  nature  of  the  accelerator. 

Aniline.  Aniline,  or  phenylamine  (commonly  called  aniline 
oil) ,  is  colorless  when  freshly  distilled,  but  on  standing,  acquires 
a  deep  red  color,  and  this  is  the  condition  in  which  it  is  found 
commercially.  It  is  an  oily  liquid,  specific  gravity  1.02,  boiling 
point  184.4C,  melting  point  -6C.  The  melting  point  is  a  par- 
ticularly useful  test  for  purity. 

•This  reaction  between  sulfur  monochloride  and  water  will  no  doubt  explain 
a  considerable  amount  of  the  trouble  experienced  with  acid  splices,  and  acid  cured 
goods  in  general,  especially  in  the  hot,  sultry  days  in  summer.  The  evaporation 
of  the  solvent  of  a  cement  cools  the  surface  below  the  dew  point,  resulting  in  a 
deposit  of  a  film  of  moisture.  The  latter  reacts  with  the  S2C12,  reducing  the 
amount  of  the  active  vulcanizing  substance  which,  in  extreme  cases,  may  be 
entirely  destroyed  before  any  vulcanization  has  taken  place. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      39 

Hexamethylenetetramine.  A  white  crystalline  powder,  com- 
monly called  hex,  or  hexa,  melting  point  about  280C,  but  decom- 
poses below  its  melting  point.  Specific  gravity  1.25.  It  is  quite 
soluble  in  water,  and  slightly  so  in  95%  alcohol. 

Thiocarbanilide.  Thiocarbanilide,  diphenylthiourea,  CS 
(NHPh)2,  commonly  called  thio,  crystallizes  in  white  plates, 
M.P.  154C,  specific  gravity  1.32.  It  is  made  by  heating  carbon 
bisulfide  with  aniline.  The  commercial  product  is  usually  a  gray 
powder,  and  may  contain  small  amounts  of  sulfur.  There  are 
at  least  a  dozen  trade  names  for  this  one  accelerator,  some  of 
the  preparations  being  a  mixture  of  thio  with  inert  pigments. 

Diphenylamine.  Diphenylamine,  or  phenyl-aniline,  NHPh2, 
has  a  molecular  weight  of  169,  specific  gravity  1.16,  melting  point 
54C,  boiling  point  302C.  It  is  only  slightly  soluble  in  water. 

Dimethylaniline.  Dimethylaniline,  PhNMe2,  is  a  yellow  liquid, 
specific  gravity  0.958,  melting  point  2.5C,  boiling  point  194C.  It 
is  very  slightly  soluble  in  water. 

Aldehyde  Aniline.  If  well  cooled  formaldehyde  is  mixed  with 
aniline,  anhydroformaldehyde-aniline  (or  trimethylenetrianiline) 
is  formed,  melting  point  140C.  In  alkaline  solution,  at  ordinary 
temperatures,  formaldehyde  and  aniline  give  methylene-diphenyl- 
diamine,  CH2(NHPh)2,  melting  at  65C.  This  may  also  be  pre- 
pared by  heating  anhydroformaldehyde-aniline  with  alcoholic 
aniline  to  100C. 

Commercial  aldehyde-aniline  is  a  mixture  of  several  sub- 
stances, the  proportions  varying  with  the  differences  in  the  con- 
trol during  the  process  of  manufacture. 

Ethylidene  Aniline.  Ethylidene  aniline  is  made  from  acetalde- 
hyde  and  aniline.  It  is  a  dark  reddish  liquid,  very  stiff  at 
ordinary  temperatures,  but  it  becomes  quite  fluid  at  the  usual 
working  temperatures  of  the  mixing  mill  (175F-200F). 

P-nitrosodimethylaniline.  P-nitrosodimethylaniline  is  obtained 
in  the  form  of  large  green,  glistening  leaflets,  melting  point  85C. 
It  stains  paper  or  cotton  a  deep  yellow.  With  caustic  alkali,  it 
breaks  down  into  nitrosophenol  and  dimethylamine,  a  reaction 
of  much  interest  in  connection  with  the  preparation  of  the  dithio- 
carbamates. 

Other  Aniline  Derivatives.  There  are  some  other  derivatives  of 
aniline  which  might  be  included  here,  but  are  not  because  they 


40  THE  ANALYSIS  OF  RUBBER 

are  of  no  importance  commercially.  We  may  mention  p-pheny- 
lenediamine,  p-aminodimcthylaniline,  etc. 

J)iph<  nijlyuanidinc.  Diphcnylguanidinc,  N11:C:  (NHPh)2, 
melting  j)oint  147C.  Jt  is  a  mono-acid  base;  witli  carbon  bisul- 
fide, it  form*  thiocarbanilide  and  thiocyanic  acid.  One  com- 
mercial preparation  consists  of  two  thirds  diphenylguanidine,  and 
one  third  magnesium  oxide. 

Triphenylguanidinc.  Two  triphenylguanidines  are  known; 
(a)  PhN:C:  (NHPh)2,  is  most  easily  prepared  by  heating  thio- 
carbanilide and  aniline,  and  distilling  off  the  excess  of  aniline. 
Hydrogen  suliide  splits  off  during  the  reaction.  This  is  the  tri- 
phenylguanidine  commonly  used  in  rubber  compounding.  When 
pure,  it  exists  as  white  crystals,  but  the  commercial  product  is 
frequently  colored  yellow  owing  to  the  excess  of  aniline  which 
has  not  been  distilled.  It  has  a  melting  point  of  143C.  (b)  The 
second  triphenylguanidine  is  derived  from  the  HC1  salt  of 
diphenylamine  and  cyananilide,  the  formula  being  NH:C. 
(PhNH).(Ph2N).  It  also  has  accelerating  properties. 

Diphenylcarboimide.  Diphenylcarboimide,  C13H10X2;  if  tri- 
phenylguanidine is  heated  under  reduced  pressure,  aniline  is  given 
off  and  diphenylcarboimide  produced,  PhN:C:NPh.  The  crude 
substance  is  glassy,  resinous,  amorphous,  with  no  definite  melting 
point,  but  softens  gradually  as  it  is  heated.  The  pure  substance 
is  said  to  have  a  melting  point  of  160C-170C. 

Aldehyde  Ammonia.  When  formaldehyde  combines  with  am- 
monia, instead  of  following  the  usual  procedure,  we  get  hexa- 
rnethylenetetramine.  Aldehyde  ammonia  is  the  product  of  the 
combination  of  acctaldehyde  and  ammonia;  Me.CHOH.NH2; 
melting  point  70C-80C,  boiling  point  100C.  It  occurs  as  color- 
less crystals,  turning  dark  on  exposure  to  the  air;  probably  on 
account  of  the  reaction  with  the  moisture  in  the  air,  since  in 
contact  with  water  it  forms  hydroacetamide.9 

Furfuramide.  Furfuramide,  formed  by  the  action  of  ammonia 
on  furfuraldehyde;  a  light  brown  crystalline  substance,  melting 
point  117C. 

Quinoidine.  The  product  sold  commercially  under  the  name 
quinoidinc,  is  the  residue  remaining  after  the  removal  of  the 
alkaloids  quinine,  cinchonine,  and  cinchonidine,  from  the  extract 
of  Peruvian  bark.  It  is  a  dark  brown  to  black  resinous  solid, 

*  Kichtor's  Organic   Chcnii.st  ry,    I  ransla  t  ion   by   E.   F.   Smith,    II,   p.  20G. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      41 

non-crystalline,  which  softens  readily,  and  mixes  well  with 
rubber. 

Piperidine.  Piperidine  is  a  colorless  liquid,  with  a  peculiar 
odor  slightly  resembling  that  of  pepper;  strongly  basic,  soluble 
in  alcohol  and  water;  boiling  point  106C.  It  is  found  in  nature 
in  combination  with  piperic  acid,  as  the  alkaloid  piperine,  or 
piperyl-piperidine,  crystallizing  in  prisms,  melting  point  129C. 
Piperine  is  chiefly  of  interest  in  combination  with  carbon  disulfide, 
when  it  forms  one  of  the  ultra-rapid  accelerators  (see  following) . 

The  So-Called  "Ultra-rapid"  Accelerators.  The  combination 
of  carbon  bisulfide  with  secondary  amines  such  as  dimethylamine, 
piperidine,  piperine,  pyrrolidine,  etc.,  gives  rise  to  the  formation 
of  substances  which  are  extremely  powerful  accelerators  of  vul- 
canization; these  are  believed  to  be  salts  of  dithiocarbamic  acid, 
and  the  accelerators  of  this  class  are  usually  called  the  thiocar- 
bamates.  They  are  so  much  more  powerful  than  the  organic 
accelerators  that  some  have  attempted  to  distinguish  them  by 
the  name  of  "ultra-rapid  accelerators." 10 

The  dithiocarbamates  are  mono-basic,  and  with  zinc  form  salts 
which  form  a  second  class  of  rapid  accelerators.  • 

A  third  class  of  rapid  accelerators,  the  thiurams,  is  formed  by 
the  oxidation  of  the  dithiocarbamates ;  the  product  is  a  derivative 
of  thiuramdisulfide,  NH2C-S-S-S-S-CNH2 ;  for  example,  the 
tetraethyl  derivative  would  be  (CSNEt2)2S2,  a  white  crystalline 
substance,  with  a  melting  point  of  70C.  A  few  of  these  products 
may  be  mentioned  as  follows : 

Dimethylamine  and  carbon  bisulfide;  C8H14N2S2,  m.p.  103C. 

Diethylamine  and  carbon  bisulfide,  C9H22N2S2;  m.p.  130C. 

Thiuramdisulfide ;  NH2CS .  S .  S .  SC .  NH2. 

Tetramethylthiuram  disulfide,  (CSNMe2)2S2;  m.p.  -   — . 

Tetraethylthiuram  disulfide,  (CSNEt2)2S2;  m.p.  70C. 

The  above  list  includes  practically  all  of  the  organic  accelera- 
tors which  have  reached  any  commercial  significance,  and  per- 
haps a  few  that  have  not  as  yet.  There  is  still  the  derivatives  of 
quinoline,  pyrrole,  piperidine,  and  many  others.  In  fact,  it  may 

10  Some  Idea  of  their  power  to  accelerate  vulcanization  may  be  gleaned  from 
the  fact  that  a  mixture  of  50  parts  each  of  rubber  and  zinc  oxide,  3  parts  of 
sulfur,  and  only  0.1  part  of  the  dimethyldithiocarbamate,  will  reach  its  maxi- 
mum cure  in  three  minutes.  Some  of  the  others  in  this  class  are  even  more 
rapid  in  this,  giving  good  cures  in  one  minute,  with  slabs  about  one  sixteenth 
of  an  inch  thick,  hardly  time  enough  for  the  heat  to  penetrate  to  the  center 
of  the  sheet. 


42  THE  ANALYSIS  OF  RUBBER 

not  be  going  too  far  to  say  that  any  basic  organic  compound, 
containing  amino,  or  imino  nitrogen,  is  a  promising  substance  in 
which  to  look  for  accelerating  properties. 

Inorganic  Accelerators. 

The  inorganic  accelerators  are  practically  limited  to  com- 
pounds of  two  elements,  lead  and  magnesium.  Calcium  hydrox- 
ide has  accelerating  power,  but  it  can  be  used  in  such  small 
quantities,  on  account  of  its  hardening  effect  on  a  compound, 
that  sufficient  of  it  cannot  be  used  to  completely  accelerate  the 
cure.  Sodium  hydroxide  in  small  amounts  acts  as  an  accelerator, 
while  in  amounts  in  the  neighborhood  of  5%,  it  actually  retards 
vulcanization.  The  lead  compounds  are  litharge,  red  lead,  basic 
lead  carbonate,  sublimed  white  lead,  sublimed  blue  lead,  and 
lead  oleate.  Magnesium  oxide  and  carbonate  are  the  only  mag- 
nesium compounds. 

Litharge. 

Litharge  should  be  clean,  dry,  pale  yellow  in  color,  free  from 
copper;  specific  gravity  9.37.  There  should  be  only  small 
amounts  of  the  dioxide.  Litharge  is  used  in  quantities  of  from 
5%  to  20%.  Of  special  interest  is  the  manufacture  of  aprons 
for  the  protection  of  workers  with  radio-active  substances.  These 
contain  about  90%  of  litharge,  9%  of  rubber,  and  1%  of  sulfur, 
by  weight. 

Tests  for  Litharge. 

Moisture.  Dry  2  gr.  of  the  sample  at  105C  for  2  hours,  cool 
and  weigh. 

Lead  Dioxide.11  Treat  1  gr.  of  the  sample  in  a  beaker 
with  15  cc.  of  nitric  acid,  sp.g  1.20.  Stir  the  sample  until  all 
trace  of  red  color  has  disappeared.  Add  from  a  calibrated  pipette 
or  burette  exactly  10  cc.  of  dilute  hydrogen  peroxide  (1  part 
of  3%  hydrogen  peroxide  to  3.5  parts  of  water).  Add  about 
50  cc.  of  hot  water,  and  stir  until  all  of  the  lead  dioxide  has  passed 
into  solution.  In  the  case  of  some  coarsely  ground  oxides,  the 
contents  of  the  beaker  may  have  to  be  heated  gently  to  effect 
complete  solution.  After  the  oxide  has  gone  into  solution  com- 
pletely, dilute  with  hot  water  to  250  cc.,  titrate  with  potassium 

"The  Chemical  Analysis  of  Lead  and  its  Compounds,  by  John  A.  Scbaeffer 
and  Bernard  S.  White,  pub.  by  Picher  Lead  Co..  Joplin,  Mo. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      43 

permanganate  solution  having  an  iron  value  of  about  .005.  Run 
a  blank  on  the  hydrogen  peroxide. 

If  the  permanganate  has  been  standardized  in  terms  of  iron, 
it  can  be  calculated  to  lead  dioxide,  using  the  factor  2.134.  From 
this  the  total  weight  of  the  dioxide  can  be  calculated. 

Copper.  Dissolve  20  gr.  of  litharge  in  dilute  nitric  acid,  and 
boil  until  solution  is  complete.  Add  40  cc.  dilute  sulfuric  acid, 
boil  gently  for  one  hour,  and  allow  to  cool.  Filter  off  the  lead 
sulphate  and  wash  thoroughly.  Nearly  neutralize  the  acid  with 
ammonia,  make  acid  with  hydrochloric  acid,  warm  the  solution, 
and  pass  in  hydrogen  sulfide.  Filter  the  precipitate,  without 
washing,  using  some  of  the  filtrate  to  transfer  the  last  traces  of 
sulfide  to  the  paper.  Dissolve  in  nitric  acid,  and  wash  the  paper 
thoroughly  with  hot  water.  Add  3  cc.  of  cone,  sulfuric  acid, 
evaporate  until  the  fumes  of  sulfuric  acid  are  evolved,  cool, 
dilute,  and,  after  standing,  filter  again,  washing  with  hot  water 
containing  a  little  sulfuric  acid.  Precipitate  the  copper  in  the 
filtrate  as  sulfide  in  an  ammoniacal  solution,  filter,  ignite  and 
weigh  in  a  covered  porcelain  crucible.  The  residue  will  be  a  mix- 
ture of  CuO  and  Cu2S.  Since  the  percentage  of  copper  is  the 
same  in  both  cases,  calculate  to  copper  using  the  factor  0.7988. 

Fineness.  Determine  the  residue  on  a  200  mesh  screen,  using 
water  to  wash  the  pigment  through,  and  breaking  up  any  loose 
lumps  with  a  rubber  policeman. 

Red  Lead. 

Red  lead  is  a  mixture  of  the  monoxide  and  dioxide,  with  a 
specific  gravity  of  9.07.  It  should  have  a  bright  red  color,  be 
clean  and  dry.  The  moisture,  lead  dioxide,  copper  and  fineness 
may  be  determined  as  under  litharge. 

White  Lead. 

White  lead  is  the  basic  carbonate,  containing  about  80% 
metallic  lead,  and  20%  of  carbon  dioxide  and  combined  water. 
The  specific  gravity  is  6.46. 

Tests  for  White  Lead. 

Total  Lead.12  Weigh  1  gr.  of  the  sample,  moisten  with  water, 
dissolve  in  acetic  acid,  and  filter,  ignite  and  weigh  the  impurities. 
Add  to  the  filtrate  25  cc  sulfuric  acid  (1-1),  evaporate  until 

"  P.  H.  Walker,  Bull.  109,  Bureau  of  Chemistry,  U.  S.  Dept.  of  Agriculture. 


44  THE  ANALYSIS  OF  RUBBER 

(lit1  acetic  ariil  is  driven  off;  cool  and  dilute  to  200  cc.  with  water, 
add  20  cc.  ethyl  alcohol,  allow  to  stand  for  2  hours,  filter  on 
a  Gooch  crucible,  wash  with  1%  sulfuric  acid,  ignite  and  weigh 
as  lead  sulfate.  Calculate  to  lead  with  the  factor  0.6829  or  to 
the  basic  carbonate  by  0.8526. 

Carbonic  Acid.  A  1  gr.  sample  is  placed  in  a  flask  containing 
a  side  arm  delivery  tube  connected  with  a  train  consisting  of 
two  U-tubes  containing  sulfuric  acid  and  potassium  bichromate, 
two  U-tubes  containing  soda-lime,  and  the  fifth  U-tube  contain- 
ing the  same  solution  as  the  second  sulfuric-bichromate  tube. 
Add  dilute  nitric  acid,  and  sweep  out  the  liberated  carbon  dioxide 
with  a  current  of  air  which  has  been  freed  from  carbon  dioxide 
by  passing  over  soda-lime.  Weigh  the  two  soda-lime  tubes, 
and  the  fifth  tube,  containing  sulfuric  acid-bichromate;  the  in- 
crease in  weight  is  carbon  dioxide. 

Fineness.    Treat  as  under  litharge. 

Sublimed  White  Lead. 

Commercial  sublimed  white  lead  is  a  basic  sulfate,  containing, 
on  an  average,  of  about  78.5%  of  lead  sulfate,  16%  of  lead  oxide, 
and  5.5%  of  zinc  oxide.  It  has  a  specific  gravity  of  6.20.  It 
should  pass  through  a  200  mesh  screen  without  appreciable 
residue. 

Sublimed  white  lead  is  used  for  its  accelerating  properties, 
which  are  almost  entirely  dependent  upon  the  content  of  lead 
oxide.  A  test  mix  would  undoubtedly  be  the  best  method  for 
testing;  the  lead  oxide  may  be  calculated  by  determining  the 
total  sulfur  and  total  lead,  and  after  calculating  the  sulfur  to 
lead  sulfate  (he  excess  of  lead  may  be  calculated  to  lead  oxide. 

Sublimed  Blue  Lead. 

Sublimed  blue  lead  contains  lead  sulfate,  sullide,  sulfite,  oxide, 
;ind  zinc  oxide,  with  occasional  traces  of  carbon.  The  fineness 
and  accelerating  properties  are  the  only  elements  of  interest; 
the  specific  gravity  will  be  about  6.50  to  7.0. 

Lead  Oleate. 

Lead  oleate  is  a  yellowish  soft  waxy  solid,  used  to  replace 
litharge  because  of  the  case  with  which  it  may  be  distributed  in 
a  rubber  mixing.  The  specific  gravity  is  1.50.  It  is  claimed  that 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      45 

the  lead  oleate  is  much  less  harsh  in  its  action  than  litharge,  with 
less  danger  of  burning  the  stock. 

Magnesium  Oxide. 

Magnesium  oxide,  MgO,  is  sometimes  called  calcined  magnesia 
from  its  method  of  preparation;  it  exerts  a  considerable  influ- 
ence on  the  vulcanization  of  rubber,  although  less  than  that  of 
litharge.  It  is  prepared  by  precipitation  as  the  carbonate,  and 
the  latter  ignited.  It  usually  contains  some  calcium  carbonate, 
but  the  amount  must  be  kept  very  low  in  order  not  to  interfere 
with  its  accelerating  power.  It  has  a  specific  gravity  of  from  3.20 
to  3.45. 

The  calcium  carbonate  may  be  determined  by  solution  of  the 
sample  in  hydrochloric  acid,  and  the  separation  of  the  calcium 
as  oxalate  from  an  ammoniacal  solution,  with  ammonium 
oxalate.  The  calcium  may  then  be  determined  in  any  desired 
way. 

Because  of  its  effect  on  the  action  of  certain  organic  accel- 
erators, magnesium  oxide  is  sometimes  used  in  amounts  of  0.25% 
to  1.0%,  in  which  case  the  accelerating  effect  of  the  magnesium 
oxide  so  used  is  small  compared  with  that  of  the  activated  or- 
ganic accelerator.  As  the  principal,  if  not  the  only  accelerator, 
it  will  be  found  in  amounts  up  to  10%. 

Magnesium  Carbonate.  Magnesium  carbonate  is  a  light,  white 
powder,  existing  in  a  finer  state  of  division  than  the  oxide;  its 
specific  gravity  is  around  2.22.  It  may  also  contain  calcium 
carbonate,  which  may  be  determined  as  under  magnesium  oxide. 

The  carbonate  is  not  as  powerful  an  accelerator  as  the  oxide, 
and  hence  will  be  found  in  somewhat  larger  amounts;  it  is  sel- 
dom used  in  less  than  amounts  around  5%,  and  may  go  as  high 
as  20%. 

In  the  absence  of  any  appreciable  amounts  of  calcium,  deter- 
mine the  magnesia  content  of  the  dry  pigment  by  igniting  to  a 
dull  red  heat,  to  constant  weight,  taking  care  that  the  residue  is 
cooled  in  a  desiccator,  and  weighed  in  a  stoppered  weighing 
bottle,  in  order  to  prevent  reabsorption  of  moisture. 

Inorganic  Fillers. 

Aluminum  Flake.  Aluminum  flake  is  essentially  a  mixture  of 
hydrated  aluminium  oxide  and  silicate.  It  is  a  white  powder, 


46  THE  ANALYSIS  OF  RUBBER 

with  a  specific  gravity  of  from  2.58  to  2.65;  with  2.60  as  a  fair 
average  of  the  commercial  lots.  It  contains  very  little  moisture 
which  may  be  driven  off  by  heating  at  100C.  Continued  ignition 
at  a  dull  red  heat  shows  an  ignition  loss  of  about  12% ;  the  resi- 
due is  the  oxide  and  silicate.  The  ignited  oxide  is  difficult  to 
get  into  solution  in  hydrochloric  acid,  even  when  fused  for 
a  short  time  with  sodium  carbonate.  This  fact  is  important, 
both  in  the  examination  of  the  pigment,  and  in  the  analyses  of 
ash. 

On  account  of  its  low  gravity  and  fineness,  it  is  used  to  replace 
some  of  the  zinc  oxide  in  a  compound,  although  it  does  not  give 
as  good  tensile  properties. 

Ammonium  Carbonate. 

Commercial  ammonium  carbonate  is  a  mixture  of  the  car- 
bonate and  carbamate;  it  is  used  to  supply  the  gas  for  making 
sponge  rubbers. 

Asbestine. 

Asbestine  is  the  trade  name  for  a  fairly  pure  magnesium  sili- 
cate, specific  gravity  2.60-2.80.  It  is  used  at  times  in  place  of 
talc  for  dusting  stocks,  and  replaces  whiting  in  some  mixes.  It 
is  a  cheap  filling  material. 

Barytes. 

Barium  sulfate  is  used  under  various  trade  names,  barytes, 
blanc  fixe,  basofor,  barium  dust,  etc.  Wiegand  has  shown  that 
this  pigment  is  a  mere  diluent;  it  is  inert  during  vulcanization. 
On  account  of  the  crystalline  nature  of  this  pigment,  it  is  not 
very  well  adapted  for  some  lines  of  manufacture,  but  finds  ex- 
tensive use  in  mechanical  goods.  The  specific  gravity  runs  be- 
tween 4.2  and  4.5.  It  should  be  free  from  grit  and  should  leave 
no  residue  on  a  200  mesh  screen.  Some  preparations  of  barytes 
are  claimed  to  have  less  than  1%  of  residue  on  a  300  mesh  screen. 
The  best  means  for  telling  the  relative  value  of  the  various 
brands  of  barytes  is  by  means  of  vulcanization  tests  with  experi- 
mental batches. 

Since  barytes  is  used  merely  as  a  filler,  it  is  seldom  found  in 
amounts  under  10%,  and  there  may  be  as  high  as  30%  in  the 
compound. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      47 

Brown  Pigments. 

The  principal  brown  pigments  are  the  various  mixtures  of 
iron  and  manganese  oxides,  the  umbers.  These  are  usually  higher 
in  manganese  than  the  siennas.  They  should  be  tested  for  grit, 
and  for  change  of  color  when  heated. 

Recent  research  has  seemed  to  indicate  that  manganese  is  re- 
sponsible for  rapid  deterioration  of  some  rubber  compounds; 
should  this  be  substantiated  with  further  work,  it  would  seem 
to  show  that  the  manganese  browns  should  be  used  with  caution, 

Calcium  Sulfate. 

Calcium  sulfate  is  rarely  used  as  such  in  rubber  compounding, 
but  it  exists  as  a  part  of  many  lots  of  commercial  golden  and 
crimson  sulfides  of  antimony. 

Chinese  Blue. 

Blue  is  not  a  color  which  is  used  to  any  very  great  extent  in 
rubber  manufacture.  The  chief  blues  are  Chinese  blue,  ultra- 
marine blue,  and  the  blue  organic  dyes. 

Chinese  (or  Prussian)  blue,  is  precipitated  from  a  mixture  of 
potassium  ferrocyanide  and  ferric  sulfate.  It  is  an  excellent  blue 
color,  but  has  limited  possibilities  in  rubber,  owing  to  its  turning 
brown  when  mixed  with  alkalies,  forming  ferric  oxide,  and  salts 
of  hydrocyanic  acid. 

Crimson  Antimony. 

Crimson  antimony  is  largely  an  oxide  or  oxysulfide  of  anti- 
mony, with  a  deep  crimson,  or  red  color;  specific  gravity  varies 
from  3.9  to  4.2.  It  is  usually  lower  in  free  sulfur  than  golden 
sulfide,  and  is  used  chiefly  on  account  of  its  color. 

Dyes. 

The  organic  dyes  are  found  chiefly  in  the  sulfur  chloride,  or 
acid,  cured  goods.  Practically  none  of  them  are  water  soluble, 
but  most  of  them  can  be  leached  out  with  alcohol,  acetone,  or 
benzene.  The  identification  of  these  dyes  is  an  exceedingly  diffi- 


48  THE  ANALYSIS  OF  RUBBER 

cult  proposition;  they  are,  as  a  rule,  merely  coloring  materials, 
and  have  no  other  effect  on  the  rubber,  so  that  any  dye  which 
will  give  the  same  color  is  no  doubt  of  equal  value,  and  the 
positive  identification  of  any  one  particular  dye  is  not  often  a 
matter  of  interest. 

Fossil  Flour. 

Fossil  flour  (tripoli,  diatomaccous  earth)  consists  of  the  re- 
mains of  diatoms,  and  is  nearly  pure  silica,  with  traces  of  alkali. 
It  may  contain  considerable  moisture,  and  the  loss  in  weight  at 
1050  is  an  important  indication  of  its  availability  for  rubber 
compounding.  It  is  a  very  poor  conductor  of  heat,  and  hence 
is  frequently  used  in  steam  valves,  etc.  The  specific  gravity  is 
about  2.00. 

Gas  Black. 

( las  black  is  a  very  pure  form  of  carbon,  prepared  by  burning 
natural  gas  with  insufficient  air  for  complete  combustion.  It  is 
the  most  finely  divided  pigment  in  use  in  rubber  compounding; 
it  contains  no  oil  or  grease,  and  on  ignition  leaves  no  residue. 
It  has  a  specific  gravity  of  1.73,  or  less  than  one  third  of  that 
of  zinc  oxide,  so  that  a  pound  of  gas  black  has  more  than  three 
times  the  volume,  and  an  even  greater  proportion  of  active  sur- 
face. It  is  hygroscopic  to  a  considerable  degree1,  taking  up  mois- 
ture from  the  air  to  the  extent  of  2  or  3%. 

(las  black  should  not  be  confused  with  lamp  black,  which  is 
made  from  the  burning  of  oils,  tars,  or  resins,  also  with  insuf- 
ficient air  for  complete  combustion.  The  flame  may  impinge 
on  a  revolving  metallic  cylinder,  as  in  the  case  of  gas  black,  or 
the  oil  may  be  fired  in  a  huge  oven,  and  the  smoke  carried 
through  a  series  of  chambers,  thus  making  a  partial  separation 
of  the  different  grades  of  black.  Those  nearest  the  fire  are,  of 
course,  heavier,  and  contain  a  larger  percentage  of  oil  than  the 
black  contained  in  those  chambers  furthest  away  from  the  fire. 
These  lamj)  blacks  are  further  purified  by  heating,  with  the  ex- 
clu-ion  of  air,  thus  reducing  the  percentage  of  oil.  Lamp  black 
is  not  as  fine  a  pigment  as  gas  black,  and  does  not  give  the  same 
improvement  in  tensile,  properties  that  the  latter  does;  in  fact, 
in  thi-  respect,  it  is  rated  below  xinc  oxide.  It  has  the  same 
specific  gravity  as  gas  black. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      49 

The  only  tests  for  gas  black,  or  lamp  black,  are  moisture,  oils, 
and  ash.  Moisture  should  be  determined  on  a  1  gr.  sample  by 
heating  to  105C,  cooling  in  a  desiccator,  and  weighing  in  a  stop- 
pered weighing  bottle.  Oil  is  determined  by  extraction  of  a  5  gr. 
sample  with  ethyl  ether,  and  weighing  the  residue.  Not  less 
than  5  gr.  should  be  taken  for  the  ash,  and  if  the  residue  is  an 
appreciable  amount,  it  shows  an  admixture  of  other  blacks,  or 
dirt. 

Owing  to  its  low  gravity,  and  fineness  of  particle  size,  gas 
black  seldom  runs  higher  than  10%,  although  there  have  been 
commercial  articles  manufactured  containing  17-20%. 

Golden  Antimony. 

Antimony  sulfide,  or  golden  sulfide,  is  a  mixture  of  the  tri-  and 
penta-sulfides  of  antimony,  free  sulfur,  and  it  may  contain  little 
or  much  calcium  sulfate.  The  pigment  varies  from  orange  to  a 
reddish  color,  the  red  being  due  to  the  oxide  or  oxysulfide.  The 
composition  varies  within  wide  limits,  as  is  shown  by  the  varia- 
tion in  the  specific  gravity  of  from  2.5  to  2.9.  It  is  not  an  accel- 
erator of  vulcanization;  its  real  value  consists  in  its  ability  to 
give  up  the  free  sulfur  to  rubber  during  vulcanization  and  yet, 
afterwards,  to  remain  free  from  blooming.  The  free  sulfur 
should  run  about  17%,  and  the  calcium  sulfate  should  be  low. 
Gaspari  gives  some  figures  showing  that  the  free  sulfur  may  vary 
from  7  to  19% ;  the  calcium  sulfate  from  3  to  50% ;  and  the  anti- 
mony sulfides  from  30  to  90%. 

When  used  for  coloring  only,  golden  sulfide  may  be  used  only 
to  the  extent  of  1  or  2% ;  when  used  as  the  source  of  sulfur  for 
vulcanization,  15  to  25%  will  be  required,  depending  largely 
upon  the  free  sulfur  and  antimony  sulfide  content  of  the  dry 
pigment. 

Tests  for  Golden  Sulfide. 

Calcium  sulfate.  Jacobson 13  recommends  the  following  simple 
test  for  calcium  sulfate:  Mix  1  gr.  of  the  original  sample 
with  2  gr.  of  sublimed  ammonium  sulfate  in  a  porcelain  crucible. 
Heat  until  the  ammonium  sulfate  and  antimony  sulfide  have  been 
driven  off;  cool  and  weigh. 

13Chem.  Ztg.  32,  984  (1908), 


50  THE  ANALYSIS  OF  RUBBER 

Free  sulfur.  Extract  1  gr.  with  acetone,  or  carbon  bisulfide,  in 
the  extractor  described  under  "acetone  extract."  Distil  off  the 
solvent,  add  100  cc.  of  water  and  3  to  5  cc.  of  bromine;  proceed 
with  the  determination  as  directed  under  the  determination  oi 
free  sulfur  in  vulcanized  articles.  Or  the  solvent  may  be  driver 
off  in  a  tared  flask,  the  flask  and  contents  dried  to  constant 
weight  at  90C,  and  the  sulfur  weighed  directly.  This  method  is 
shorter,  but  as  a  rule,  not  as  accurate.14 

Graphite. 

(irnphite,  or  plumbago,  is  a  natural  form  of  carbon,  used  tc 
some  extent  on  account  of  its  lubricating  value  in  preventing 
adhesion  between  rubber  stocks  and  metal.  It  may  be  found  ir 
some  stocks  where  its  acid  and  alkali  resisting  properties  an 
of  peculiar  value. 

Greens. 

Most  of  the  green  pigments  used  in  rubber  manufacture  ar< 
organic  colors.  Brunswick  green,  a  mixture  of  Chinese  blue  anc 
chrome  yellow  (lead  chromate),  darkens  when  heated  with  sul 
fur.  This  green  is  sometimes  marketed  as  "chrome  green/'  bui 
the  true  chrome  green  is  the  oxide  of  chromium,  Cr203,  and  is  b) 
far  the  best  mineral  green  for  rubber  work,  since  it  is  not  readih 
affected  by  heat,  acids,  or  alkalies. 

14  Luff  and  Porritt,  J.  Soc.  Chein.  Ind.  W,  275-8T  (1921),  found  by  previous!; 
heating  antimony  sulfide  before  extracting  the  free  sulfur,  the  latter  varies 
considerably,  as  will  be  seen  from  the  following  table: 

Sri.Fl'K     EXTKACTKD    FROM     ANTIMONY     SfLFIDK. 

EXTRACTION  FOR  5  HOURS  WITH  CARBON  BISULFIDE. 

Fnheated  Heated  1  to  2  hours 

1st  5  2nd  5 

Sample             hours  hours  125  C  150  C  230  < 

1                      3.70  0.33  2.99  4.88  6.94 

31.21  0.33  29.75  32.19  32.71 

3  1.02  0.13  .95  1.01  .98 

4  4.f>4  0.17  1.56  4.86  4.90 
I). 14  0.13  8.90  13.74  15.38 

The  presumption  is  that  the  sulfur  extracted  is  available  for  vulcanization 
If  during  vulrani/at  ion  a  greater  percentage  of  free  sulfur  than  that  indicate, 
at  normal  temperatures  is  available,  tbis  fact  is  of  decided  interest  and  value 
It  is  desirable  that  this  subject  be  followed  up — we  should  know  more  definitel, 
why  at,  12."C  the  free  sulfur  drops  off,  and  more  particularly  how  long,  afte 
heating,  the  additional  free  .sulfur  is  capable  of  being  extracted  with  carboi 
bisulfide, 


THE  PREPARATION  OF  RUBBER  COMPOUNDS      51 

Iron  Oxides. 

Red  oxide  of  iron  (Indian  red,  Venetian  red)  is  one  of  our 
most  valuable  pigments,  not  merely  for  its  color,  but  for  the 
valuable  tensile  properties  which  it  imparts  to  rubber,  ranking, 
in  this  respect,  not  very  far  behind  zinc  oxide.  It  is  practically 
pure  Fe203,  running  over  98%,  with  small  amounts  of  water.  It 
holds  its  color  very  well  during  vulcanization.  The  specific 
gravity  is  between  5.0  and  5.20. 

These  iron  oxides  may  be  obtained  in  a  great  variety  of  shades, 
depending  largely  on  the  method  of  preparation.  The  color 
should  always  be  matched  against  a  standard,  and  it  is  best  to 
make  a  heat  test  at  150C,  as  recommended  for  golden  sulfide. 

Lime. 

The  lime  which  we  use  is  the  air  slaked  hydroxide,  specific 
gravity  of  2.4.  It  is  used  largely  because  it  will  take  up  small 
amounts  of  moisture  which  may  be  present  in  the  compound, 
and  reduce  the  danger  of  "blowing,"  or  porosity.  It  has  a  decided 
hardening  effect  on  the  rubber,  and  hence  may  not  be  used  in 
anything  but  small  amounts.  It  also  is  believed  to  be  responsible 
for  rapid  deterioration.  It  has  some  accelerating  effect  on  the 
vulcanization,  and  due  allowance  must  be  made  for  this  factor. 

Lithopone, 

Lithopone  is  a  mixture  of  barium  sulfate  and  zinc  sulfide,  con- 
taining about  25  to  30%  of  the  latter.  It  is  not  as  fine  a  pigment 
as  the  oxide,  and  does  not  produce  as  good  tensile  properties.  It 
is  unaltered  during  vulcanization,  and  is  often  used  as  a  substi- 
tute for  the  more  expensive  zinc  oxide.  It  must  be  low  in  water 
soluble  matter,  lead,  and  chlorides.  The  specific  gravity  is  4.20. 

Tests  for  Lithopone. 

Moisture.    Heat  1  gr.  for  2  hours  at  105C,  cool  and  weigh. 

Barium  Sulfate.  To  1  gr.  of  pigment,  add  10  cc.  cone,  hydro- 
chloric acid  and  1  gr.  of  potassium  chlorate  in  small  portions. 
Evaporate  to  half  its  volume,  add  100  cc.  of  hot  water,  and  a 


52  THE  AXALYSIS  OF  RUBBER 

few  re.  of  dilute  sulfuric  acid.  Boil  and  filter,  wash  thoroughly, 
ignite  and  weigh  the  barium  sulfate.  Any  siliea,  and  some  of 
the  alumina,  it'  present,  would  be  included,  but  it  is  not  worth 
attempting  to  make  a  separation. 

Total  zinc.15  Take  1  gr.  of  the  pigment,  and  boil  with  the 
following  solution:  Water  30  ce.,  ammonium  chloride  4  grams, 
cone,  hydrochloric  acid  0  re.  Dilute  to  200  er.  with  hot  water; 
add  2  cc.  of  a  saturated  solution  of  sodium  thiosulfate,  and  titrate 
with  a  standard  solution  of  potassium  ferrocyanide,  using  5'., 
uranium  nitrate  as  an  outside  indicator.  Calculate  the  zinc  to 
zinc  sulfide.16 

Fineness.  Lithopone  should  leave  practically  no  residue  on  a 
200  mesh  screen. 

Sodium  Bicarbonate. 

Sodium  bicarbonate  is  used  in  sponge  rubber,  since  on  heating 
it  breaks  down  into  the  carbonate,  carbon  dioxide,  and  water, 
in  the  vulcanized  article,  it  is  found  chiefly  as  the  carbonate, 
Na2C03. 

Talc. 

Talc  is  used  extensively  as  a  lubricant,  to  prevent  rubber  sur- 
faces from  sticking  together,  and  in  molds,  to  prevent  the  rubber 
stocks  from  sticking  to  the  mold.  It  is  rarely  used  as  a  filler, 
but  rubber  has  such  a  facility  for  absorbing  talc  that  the  analyst 
will  rarely  fail  to  find  1%  or  2%  of  talc  in  vulcanized  compounds. 
The  specific  gravity  is  about  2.7,  and  the  color  will  vary  from  a 
brilliant  white  to  a  dirty  gray. 

Talc  usually  has  a  considerable  amount  of  grit,  largely  sand 
and  the  iron  minerals  which  are  usually  found  associated  with 
talc  (pyroxene,  hornblende  and  biotite). 

Ultramarine. 

Ultramarine  is  probably  a  double  silicate  of  sodium  and 
aluminium,  with  some  sodium  sulfide.  The  sulfide  seems  an 
essential  part;  at  least,  if  treated  with  acids,  hydrogen  sulfide  is 
given  off  and  the  blue  color  fades  out.  It  is  the  best  known  blue 

:'  Low's  Technical  .Methods  of  Ore  Analysis,  p.   284. 

'"  There  is  u  slight  error  here,  owing  to  the  fact  that  part  of  the  zinc  Is 
present  as  (he  oxide,  hut  the  error  is  usually  negligible. 


THE  PREPARATION  OF  RUBBER  COMPOUNDS     53 

pigment  for  hot  vulcanization,  but  it  is  not  safe  to  use  it  in  goods 
for  acid  curing,  since  sulfur  chloride  usually  contains  free  acid, 
and  the  latter  would  react  with  the  ultramarine,  and  either  par- 
tially or  wholly  destroy  the  color.  The  specific  gravity  is  2.35. 

Vermilion. 

The  true  vermilion  is  the  sulfide  of  mercury,  a  very  heavy 
pigment,  specific  gravity  of  about  8.00,  but  possessing  a  brilliant 
red  color.  It  is  the  most  expensive  pigment  used  in  commercial 
rubber  goods,  and  since  its  color  is  its  only  good  point,  it  is  sel- 
dom worth  what  it  costs,  and  is  not  likely  to  be  encountered  by 
the  average  analyst.  Some  so-called  vermilions  are  merely  red 
lakes.  In  the  dry  pigment,  they  are  easily  recognized  by  the 
difference  in  gravity. 

Whiting. 

On  account  of  its  low  cost,  whiting  is  extensively  used.  It  is 
essentially  calcium  carbonate,  and  should  be  entirely  soluble  in 
dilute  acids,  and  should  contain  no  free  alkali.  It  is  somewhat 
hygroscopic,  specific  gravity  2.67,  and  contains  small  amounts 
of  iron,  alumina,  and  silica.  It  may  be  found  in  any  amount  up 
to  say  25  or  30%. 

Tests  for  Whiting. 

Moisture.    Heat  2  gr.  for  2  hours  at  105C;  cool  and  weigh. 

Free  Alkali.  In  an  Erlenmeyer  flask,  shake  10  gr.  of  pigment 
with  100  cc.  of  water,  add  a  few  drops  of  phenolphthalein;  the 
color  should  not  be  deeper  than  a  faint  pink. 

Water  Soluble.  Heat  10  gr.  of  pigment  with  100  cc.  of  distilled 
water,  filter,  evaporate  to  dryness  in  a  weighed  beaker  or  dish, 
heat  to  105C  for  15  minutes,  cool  and  weigh. 

Fineness.  Whiting  should  leave  practically  no  residue  on  a 
200  mesh  screen. 

Yellow  Ochre. 

The  yellow  ochres  are  practically  all  clays,  containing  large 
amounts  of  hydrated  iron  oxide;  the  specific  gravity  will  vary 
enormously,  probably  more  than  any  other  pigment,  from  say 
3.50  to  5.00.  The  higher  gravity  ochres  are  considered  better 


54  THE  ANALYSIS  OF  RUBBER 

for  the  purpose;  they  hold  their  eolor  better,  have  a  stronger 
color,  and  are  less  likely  to  change  color  during  vulcanization. 
The  stronger  colored  ochres  are  to  be  preferred  also,  because  less 
is  required  to  give  a  definite  color  in  the  finished  article. 

Zinc  Oxide. 

Zinc  oxide  is  unquestionably  the  most  widely  used  pigment 
in  rubber  manufacture.  Its  extreme  fineness  makes  it  particu- 
larly valuable  where  strength  and  wear-resisting  qualities  are 
desired,  it  is  unaffected  in  color  during  vulcanization,  and  hence 
can  be  used  in  any  color  combination.  It  has  a  special  field  of 
usefulness  in  that  it  also  provides  a  rubber  mix  with  an  alkaline 
reaction,  which  permits  many  of  the  organic  accelerators  to  func- 
tion. Thiocarbanilide,  the  dithiocarbamates,  thiurams,  etc.,  will 
not  accelerate  vulcanization  unless  the  mixture  is  basic,  and  zinc 
oxide  answers  the  purpose  in  a  most  acceptable  manner. 

With  some  accelerators,  zinc  oxide  reacts  during  vulcaniza- 
tion to  form  a  new  accelerator.  The  mechanism  of  such  reac- 
tions is  still  a  matter  under  investigation,  and  while  splendid 
results  have  been  accomplished  by  the  workers  in  this  field,  we 
can  hardly  feel  that  the  last  word  has  been  said  on  the  subject. 
Probably  the  safest  position  to  take  is  to  say  that  practically  all 
of  the  organic  accelerators  are  more  active  in  the  presence  of  a 
basic  oxide,  such  as  magnesium,  zinc  and  lead,  and  there  are 
some  which  will  not  react  without  some  such  basic  substance. 
In  a  few  cases  the  marked  difference  between  the  reaction  when 
zinc  oxide  is  present,  compared  with  some  other  basic  oxide, 
suggests  a  possible  reaction  between  the  zinc  oxide  and  the  ac- 
celerator. 

Zinc  oxide  may  be  absent  altogether,  it  may  constitute  only 
a  small  percentage  of  the  whole  compound,  or  it  may  be  as  high 
as  50%,  as  for  example,  in  some  of  the  white  tire  treads. 

Tests  for  Zinc  Oxide. 

Zinc  oxide  should  be  tested  for  moisture,  lead,  chlorides,  sul- 
fatos,  sulfides,  and  water-soluble  matter.  The  specific  gravity 
is  5.57,  and  the  fineness  such  that  there  should  be  no  residue  on 
u  200  mesh  screen,  and  very  little  on  a  300  mesh.  Over  0.1%  of 


THE  PREPARATION  OF  RUBBER  COMPOUNDS     55 

lead  renders  it  unfit  for  bright  colored  mixes,  while  much  larger 
amounts  would  so  change  the  vulcanization  as  to  prevent  its  use 
altogether,  unless,  which  seems  unlikely,  one  could  depend  upon 
getting  a  zinc  oxide  with  absolutely  constant  lead  content. 
Chlorine  is  seldom  found  in  amounts  over  0.01,  but  cases  have 
been  known  in  which  the  chlorine  ran  over  0.20%.  Such  an 
amount  will  usually  be  reflected  in  an  unusually  high  water 
soluble  extract.  Metallic  chlorides  have  a  deleterious  effect  on 
many  rubber  compounds,  especially  cements,  and  hence  the 
chlorine  content  must  be  kept  low. 

Moisture.  Dry  2  gr.  at  105C  for  2  hours,  cool,  weigh,  and  cal- 
culate the  loss  to  percentage. 

Insoluble  Matter.  In  a  250  cc.  beaker,  treat  10  gr.  with  50  cc. 
of  cone,  hydrochloric  acid;  evaporate  to  dryness,  take  up  the 
residue  with  water  and  a  few  drops  of  hydrochloric  acid,  filter, 
and  wash  thoroughly  with  hot  water.  Ignite  the  residue,  cool 
and  weigh. 

Water  Soluble.  Treat  10  gr.  with  200  cc.  of  water,  heat  on  a 
hot  plate  for  one  hour,  filter  into  a  250  cc.  graduated  flask,  cool 
to  room  temperature,  and  make  up  to  the  mark.  Take  a  50  cc. 
portion,  evaporate  to  dryness  in  a  weighed  beaker  or  dish,  dry 
for  2  hours  at  105  C,  cool  and  weigh. 

Chlorides.  From  the  water  soluble  extract  take  a  50  cc.  por- 
tion, make  slightly  acid  with  nitric  acid,  add  10  cc.  N/10  silver 
nitrate  and  a  few  drops  of  ferric  chloride;  titrate  the  excess  of 
silver  nitrate  with  standard  ammonium  thiocyanate. 

Sulfates.  To  another  50  cc.  portion  of  the  soluble  matter,  add 
several  drops  of  cone,  hydrochloric  acid,  and  heat;  add  1  cc.  of 
10%  barium  chloride  solution,  allow  to  stand  overnight;  the 
next  day,  if  there  is  any  precipitate,  it  can  be  determined  as 
usual. 

Total  Sulfur.  Treat  10  gr.  of  pigment  with  25  cc.  of  cone, 
hydrochloric  acid  and  10  cc.  bromine  water.  Evaporate  to 
dryness,  take  up  with  50  cc.  of  hot  water  and  a  few  drops  of 
hydrochloric  acid,  filter  from  any  insoluble  matter;  heat  nearly 
to  boiling,  add  1  cc.  of  10%  barium  chloride,  and  after  standing 
overnight  determine  any  barium  sulfate  which  may  have  pre- 
cipitated in  the  usual  manner. 

Lead.  The  filtrate  from  the  determination  of  insoluble  matter 
is  nearly  neutralized  with  sodium  carbonate,  and  the  lead  pre- 


56  THE  ANALYSIS  OF  RUBBER 

cipitated  with  hydrogen  sulfide.  The  qualitative  test  is  usually 
sufficient;  if  desired,  the  lead  sulfide  may  be  determined  by  any 
of  the  usual  methods. 

Fineness.  Place  10  gr.  on  a  200  mesh  screen  sieve,  and,  with 
a  gentle  current  of  water,  wash  the  pigment  through  the  screen. 
Any  loose  aggregates  may  be  broken  up  with  a  policeman.  There 
should  be  no  residue.  Repeat  with  a  300  mesh  screen,  and  if  any- 
thing remains  on  the  screen,  it  should  be  transferred  to  filter 
paper,  ignited,  and  weighed. 


Chapter  IV. 
The  Theory  of  Vulcanization. 

Having  before  us  the  proposition  that  we  are  at  this  time  pri- 
marily interested  in  the  process  of  vulcanization  as  a  change  in 
chemical  composition,  without  necessarily  dealing  with  the  man- 
ner in  which  such  a  change  takes  place,  it  seems  as  though  a 
detailed  study  of  the  various  theories  of  vulcanization  is  quite 
beyond  the  scope  of  the  present  work,  and  we  will  therefore  go 
into  the  subject  only  to  the  extent  necessary  to  develop  the  facts 
regarding  the  chemical  changes  during  this  process. 

The  term  vulcanization  has  been  used  freely,  and  it  will  no 
doubt  clarify  matters  if  we  attempt  to  define  it.1  For  our  pur- 
pose, we  will  assume  that  vulcanization  will  mean  the  addition 
of  any  element  or  group  of  elements,  of  which  we  may  use  sulfur 
and  sulfur  chloride  as  the  principal  examples,  to  crude  rubber, 
or  a  mixture  of  crude  rubber  with  other  substances,  whereby 
the  crude  rubber,  or  rubber  mixing,  is  changed  from  a  sticky, 
plastic  mass  into  a  substance  having  a  certain  degree  of  tough- 
ness, hardness,  resiliency,  and,  in  general,  such  properties  as  are 
usually  associated  with  what  we  know  as  vulcanized  rubber.  By 
this  definition,  we  purposely  avoid  including  time  or  temperature 

*It  is  of  peculiar  interest  that  while  this  book  was  in  press  an  article  by 
"Andrew  H.  King"  appeared  in  Chem.  &  Met.  Eng.  25,  1038-42  (1921),  on  "The 
Aging  of  Rubber,"  in  which  he  gave  a  definition  of  vulcanization  which  so 
nearly  parallels  our  own,  as  to  make  it  well  worth  while  quoting  what  he  has 
to  say: 

"By  vulcanization,  we  mean  the  addition  of  a  substance  or  substances  to 
rubber,  which  results  in  the  production  of  a  more  elastic  material,  i.e.,  one  with 
less  plasticity.  When  the  change  becomes  of  a  sufficient  magnitude  that  the 
product  becomes  of  commercial  value,  it  is  then  known  as  'cure.'  It  is  well 
known  that  substances  other  than  sulfur  or  sulfur  chloride — for  example, 
oxygen,  chlorine,  selenium,  etc.,  produce  a  change  in  plasticity — in  other  words, 
they  vulcanize — but  the  products  obtained  in  this  way  have  not  to  date  had  any 
commercial  value,  and  therefore  cannot  be  called  'cured.'  In  speaking  of  addi- 
tional vulcanization,  it  is  to  be  understood  that  we  are  not  limiting  ourselves 
to  sulfur  or  sulfur  chloride." 

Later  on,  in  the  same  article,  "King"  says :  "A  surface  aging  which  results 
in  hardening  or  checking  of  the  surface,  is  probably  due  largely  to  additional 
vulcanization  by  oxygen  ; — internal  aging  may  be  sulfur  and  oxygen." 

57 


58  THE  AXALYV*  OF  1WBBER 

as  a  (k'iinitc  factor  in  the  process;  nor  do  we  say  that  the 
process  can,  or  cannot  take  place  in  the  presence  or  absence  of 
substances  which  may  act  as  catalysts.  The  point  which  we  wish 
to  make  regarding  catalysts  is,  that  they  change  the  reaction  as 
regards  time,  or  temperature,  or  perhaps  both,  but  they  do  not 
change  the  principal  reaction  itself.  Taking  the  reaction  between 
sulfur  and  crude  rubber  as  an  example,  we  finally  come  to  the 
point  where  the  rubber  is  saturated  with  sulfur,  and  has  the 
formula  lC10H10S,)x.  By  the  use  of  catalysts,  we  would  get 
exactly  the  same  product,  only  in  a  shorter  time,  or  at  a  lower 
temperature.  The  use  of  these  catalysts  is  therefore  a  matter  of 
commercial  economy  of  time.  It  is  true,  when  we  use  the  longer 
processes,  or  higher  temperatures,  we  have  side  reactions,  depoly- 
merization,  etc.,  but  the  main  process  is  the  same  in  each  case. 

C.  0.  Weber  found -that  when  he  heated  rubber  and  sulfur  to- 
gether, he  obtained  a  substance  having  as  high  as  32 %  of  sulfur, 
corresponding  to  (C1UH1(.S2)X;  with  sulfur  chloride,  he  obtained 
(C10Hlt)S2Cl2)x.  He  therefore  decided  that  rubber  was  a  poly- 
prene,  and  that  it  combined  with  sulfur  to  form  a  series  of  poly- 
prene  suliides,  the  final  product  being  identified  as  above.  He 
was  unable  to  isolate  any  of  the  intermediate  products,  and  was 
obliged  to  assume  their  existence.  Ostwald,  reviewing  the  work 
of  Weber,  Holm,  Seeligmann,  Axelrod,  Hiibener,  Stern,  Hinrich- 
sen,  and  others,  came  to  the  conclusion  that  the  chemical  theory 
did  not  satisfactorily  explain  the  matter,  and  that  the  facts  as 
known  were  more  in  accordance  with  an  adsorption  process  than 
a  chemical  one.  He  based  his  conclusions  on  the  following: 
That  there  was  always  at  least  a  small  amount  of  free  sulfur  re- 
maining after  vulcanization  (but  which  we  now  know  is  not  so) ; 
that  the  process  was  a  reversible  one  and  that  the  rate  of  adsorp- 
tion depended  upon  the  amount  of  working  which  the  rubber 
sustained.  Special  emphasis  was  laid  on  the  temperature  coeffi- 
cient, 1.87,  which  seemed  to  agree  more  with  the  coefficient  for 
an  adsorption  process  than  a  chemical  one.  Ostwald  W7as  perhaps 
correct  in  saying  that  the  evidence  at  the  time  was  not  sufficient 
to  sustain  the  contention  that  the  process  was  a  chemical  one; 
on  the  other  hand,  he  himself  included  facts  which  as  Loewren  2 
has  pointed  out,  are  explainable  only  on  the  theory  of  a  chemical 
process.  In  the  preparation  of  the  bromine  and  nitrosite  deriva- 

»Z.  Angew.  Chem.  25,  1552-60. 


THE  THEORY  OF  VULCANIZATION  59 

tives  of  rubber,  it  has  been  observed  that  the  derivatives  carry 
all  of  the  combined  sulfur,  which  would  seem  to  indicate  a  chemi- 
cal bond  between  the  rubber  and  the  sulfur.  Spence  showed  that 
not  only  did  the  bromine  derivatives  carry  all  of  the  combined 
sulfur,  but  that  in  a  series  of  compounds,  the  bromine  and  sulfur 
bore  stoichiometric  relations.  Spence  found  evidence  of  an 
adsorption  effect  between  the  free  sulfur  and  the  rubber,  preced- 
ing the  actual  chemical  combination  of  the  two.  We  shall  see  in 
due  course,  when  taking  up  the  subject  of  the  direct  determination 
of  rubber,  the  importance  of  the  conclusions  which  we  reach 
regarding  the  nature  of  the  reaction  between  rubber  and  sulfur. 
Having  arrived  at  the  conclusion  that  the  reaction  is  a  chemi- 
cal one,  we  may  pass  on  to  the  mechanism  of  the  reaction.  Crude 
rubber  will  not  combine  with  sulfur  to  any  appreciable  extent  at 
ordinary  temperatures.  With  the  exception  of  what  we  have 
called  the  ultra-rapid  accelerators  (dithiocarbamates,  etc.)  there 
is  no  appreciable  reaction  until  a  temperature  of  at  least  100C 
is  obtained,  and,  for  ordinary  mixes,  the  rate  at  this  temperature  is 
exceedingly  slow.  The  ordinary  commercial  range  may  be  said  to 
be  between  125C  and  150C.  While  exact  data  are  lacking  it  has 
been  estimated  that  for  each  6  to  80  increase  in  temperature,  the 
speed  of  the  reaction  is  doubled,  i.e.,  the  time  required  for  correct 
vulcanization  is  reduced  by  one  half.3  Furthermore,  the  speed  of 
the  reaction  may  be  enormously  altered  by  the  addition  of  cat- 
alysts. It  will  be  noted  that  the  reaction  takes  place  best  when 
the  mixture  is  weakly  alkaline ;  acids,  or  strong  alkalies,  retard  or 
even  prevent  the  combination  of  rubber  and  sulfur.  About  0.1% 
of  caustic  soda  acts  as  an  accelerating  agent,  5%  retards  the 
reaction  almost  completely. 

These  accelerators  not  merely  affect  the  speed,  but  also  lower 
the  temperature  range  at  which  appreciable  vulcanization  takes 

8  Probably  every  rubber  chemist  has  some  such  formula  on  which  he  bases 
changes  in  curing,  and  while  such  figures  are  only  approximate,  they  do  give 
some  idea  of  the  order  of  magnitude  of  the  change  in  the  velocity  of  the  reac- 
tion. The  point  is  of  particular  commercial  importance,  because  it  shows  the 
necessity  for  maintaining  cures  of  rubber  articles  at  exactly  the  prescribed  time 
and  temperature.  For  example,  in  a  cure  of  60  minutes  at  140C,  an  error  of 
1C  would  be  equivalent  to  adding  from  8  to  10  minutes  to  the  regular  cure. 
Sufficient  attention  has  not  been  paid  to  this  important  question,  and  the  only 
reason  that  more  trouble  has  not  resulted  is  that  the  maximum  in  most  com- 
pounds is  not  a  point,  but  extends  over  a  range  of  some  minutes,  and  this 
automatically  provides  a  certain  tolerance.  With  rapid  curing  compounds,  how- 
ever, the  maximum  is  usually  just  a  point  in  the  curve  and  a  variation  in  the 
temperature  results  either  in  an  under,  or  overcure. 


00  THE  ANALYSIS  OF  RUBBER 

place;  some  of  them,  as  has  been  pointed  out  by  Ostromuislenskii, 
Bruni,  Bedford  and  others,  increase  the  speed  at  ordinary  tem- 
peratures to  the  point  where  it  becomes  noticeable. 

One  more  noticeable  action  with  these  organic  accelerators  is 
the  difference  in  the  change  in  the  velocity  of  the  reaction,  at 
ordinary  temperatures,  when  the  amount  of  the  accelerator  is 
varied.  For  example,  0.0orr  of  the  dimethyldithiocarbamate  may 
be  mixed  with  rubber,  sulfur,  and  zinc  oxide,  at  a  temperature 
around  100C,  for  some  time,  without  any  noticeable  effect  on  the 
rubber.  AVith  0.25^  of  the  same  accelerator,  in  a  few  minutes  a 
decided  change  takes  place,  and  the  rubber  becomes  hard  and 
unworkable,  and  is  clearly  partially  vulcanized. 

Cold  Vulcanization. 

The  acid  cure  process  of  cold  vulcanization  consists  in  sub- 
mitting rubber  to  the  action  of  sulfur  monochloride,  either  in 
vapor  form  or  in  solution.  The  reaction  is  similar  to  that  of  hot 
vulcanization;  the  sulfur  chloride  adding  at  the  double  bond, 
forming  an  addition  compound,  but  in  this  case,  both  sulfur  and 
chlorine  are  added,  and  the  resulting  compound  is  different  in 
chemical  composition,  although  greatly  resembling  the  hot  vul- 
canization product  in  many  of  the  tensile  properties.  One  im- 
portant fact  stands  out.  that  these  properties  are  not  as  lasting 
in  the  acid  cured  as  they  are  in  the  hot  vulcanized  rubber. 

The  reaction  between  rubber  and  sulfur  chloride  is  practically 
instantaneous;  consequently,  an  article  to  be  manufactured  by 
this  method  must  first  be  brought  to  its  final  form  prior  to  vulcan- 
ization. It  has  often  been  said  that  the  reaction  is  a  surface 
one,  but  this  does  not  exactly  explain  the  true  state  of  affairs. 
In  the  case  of  a  sheet  of  rubber  exposed  to  the  vapors  of  sulfur 
chloride,  the  gas  will  be  absorbed  by  the  outer  surface,  but  before 
it  can  diffuse  into  the  center  of  the  sheet,  chemical  combination 
between  the  two  takes  place.  This  will  continue  until  the  surface 
has  taken  up  all  of  the  sulfur  chloride  with  which  it  can  combine. 
In  the  meantime,  especially  if  the1  sheet  is  very  thick,  the  center 
of  the  sheet  is  unchanged.  A  somewhat  better  distribution  of  the 
Milfur  chloride  is  effected  by  swelling  the  sheet  in  solvents  like 
ben/ene  and  then  dipping  the  article  in  a  solution  of  the  sulfur 


THE  THEORY  OF  VULCANIZATION  61 

chloride  in  benzene.    In  this  way,  the  penetration  of  the  sulfur 
chloride  is  facilitated,  and  better  results  obtained. 

There  is  no  excess  of  sulfur  chloride  remaining  as  long  as  the 
rubber  is  at  all  unsaturated,  and  since  there  is  no  free  sulfur, 
acid  cured  articles  do  not  show  the  sulfur  blooming  so  common 
with  hot  vulcanized  articles. 

Vulcanization  With  Mixed  Gases. 

A  new  method  of  cold  vulcanization  through  the  interaction 
of  two  gases,  has  been  proposed  by  Peachey.4  It  consists  simply 
in  treating  a  rubber  compound  with  sulfur  dioxide,  followed  by 
hydrogen  sulfide.  Sulfur  is  liberated  in  such  an  active  form  that 
it  can  immediately  combine  with  the  rubber.  In  order  to  avoid 
the  possibility  of  having  sulfuric  acid  remain  in  the  rubber,  it  has 
been  found  advantageous  to  use  the  sulfur  dioxide  first  and 
follow  this  by  an  excess  of  hydrogen  sulfide,  since  the  latter  is 
inert,  and  will,  in  time,  be  lost  by  diffusion.  A  control  of  the 
extent  of  the  vulcanization  is  obtained  by  adding  exact  quantities 
of  sulfur  dioxide;  since  an  excess  of  hydrogen  sulfide  is  used,  the 
exact  amount  of  sulfur  to  be  added  to  the  rubber  can  be  cal- 
culated. 

Since  this  process  of  vulcanization  takes  place  at  ordinary  tem- 
peratures, there  is  no  doubt  that,  if  practicable,  it  can  be  used  with 
many  substances  as  fillers  which  it  is  not  possible  to  use  under 
present  conditions.  This  is  especially  true  of  some  of  the  bright 
organic  colors.  It  is  very  noticeable,  for  example,  that  a  much 
wider  range  and  more  brilliant  colors  may  be  used  with  sulfur 
chloride  vulcanization  than  with  the  hot  vulcanization.  It  is, 
however,  a  question  of  time  and  temperature  of  heating;  with  the 
ultra-rapid  accelerators,  it  is  quite  possible  that  this  advantage 
will  not  be  as  marked  as  it  is  with  the  much  slower  accelerators. 

4  British  patent  136,716,  Feb.  21,  1921 ;  cf.  also  Caoutchouc  and  Guttapercha 
/8,  10744-5  (1921)  ;  Dubosc  claims  that  in  a  discussion  of  the  theory  of  vul- 
canization, he  stated  that  the  reaction  may  be  caused  by  the  production  of 
colloidal  sulfur.  He  showed  tbat  sulfur  dioxide  and  hydrogen  sulfide  could  be 
generated  by  the  ingredients  of  a  rubber  compound,  and  further  stated  that  if 
hydrogen  sulfide  and  sulfur  dioxide  were  simultaneously  present,  they  would 
combine  to  liberate  sulfur  in  such  a  form  as  to  enable  it  to  immediately  combine 
with  the  rubber.  In  this  instance,  Dubosc  was  discussing  the  reaction  in  con- 
nection with  the  theory  of  hot  vulcanization,  but  the  latter  was  merely  used  as 
a  source  of  the  gases  mentioned,  and  not  necessarily  the  temperature  at  which 
the  gases  would  unite  to  give  off  sulfur  as  indicated.  Whether  or  not  this  may 
be  called  an  anticipation  of  Peach ey's  patents  remains  to  be  decided. 


02  THE  ANALYSIS  OF  RUBBER 

Ostromuislenskii's  Theories  of  Vulcanization. 

Much  has  been  said  on  the  subject  of  the  theories  of  vulcaniza- 
tion advanced  by  Ostromuislenskii,  but  if  we  can  maintain  our 
definition  of  vulcanization  given  in  the  beginning  of  this  chapter, 
we  cannot  see  that  there  exists  any  fundamental  difference  be- 
tween his  theories,  and  the  present-day  practice.  He  has  shown 
that  at  ordinary  temperatures,  he  can  cause  rubber  and  sulfur 
to  unite  in  the  presence  of  piperidyl-piperidine-dithiocarbamatc. 
With  a  sufficient  amount  of  accelerator,  the  same  thing  can  be 
done  with  dimethyldithiocarbamate,  but  if  we  reduce  the  quantity 
of  the  accelerator  to  the  neighborhood  of  0.05%,  then  we  will 
find  that  the  reaction  will  be  so  slow  at  ordinary  temperatures  as 
to  be  commercially  negligible.  It  now  becomes  merely  a  question 
of  concentration  of  accelerator  in  order  to  make  the  velocity  of 
the  reaction  at  ordinary  temperatures,  which  is  so  slow  as  to 
approach  zero,  approach  a  finite  quantity  that  will  be  visible  to 
the  eye  in  a  reasonably  short  time. 

A  much  more  distinctive  process  is  the  production  of  a  vul- 
canized rubber  substance  by  the  addition  of  trinitrobenzene; 
with  bcnzoyl  peroxide,  with  halides  and  halide  esters.5  These 
products  have  some  of  the  properties  of  rubber-sulfur  vulcani- 
zates.  However,  it  must  be  concluded  that  we  have  here  nothing 
to  invalidate  our  present  conception  of  vulcanization,  and  that 
what  has  been  accomplished  is  to  show  that  the  change  from  the 
sticky,  plastic  rubber,  which  was  first  thought  to  be  a  function  of 
sulfur,  and  was  later  extended  to  include  sulfur  chloride,  is  really 
a  property  of  a  number  of  substances.  Some  of  these  may  require 
heating,  and  some  do  not;  some  require  the  presence  of  metallic 
oxides,  and  still  others  do  not.  As  far  as  can  be  seen,  the  chief 
difference  which  has  been  noted  up  to  this  time,  is  the  stability 
(jf  the  various  products  vulcanized  in  the  different  ways,  and  it 
may  be  that  in  order  to  arrive  at  a  satisfactory  definition  of 
what  we  mean  by  vulcanization,  we  shall  not  only  have  to  state 
that  the  vulcanized  articles  shall  have  certain  definite  properties, 

"Then-  is  an  excellent,  analogy  here  between  the  various  combinations  of 
rubber  with  (.-lenient s,  or  groups  of  elements,  and  the  similar  reactions  of  the 
un saturated  fatty  acids,  such  as  linoleic,  linolenic,  etc.  With  sulfur  and  sulfur 
chloride,  we  have  products  quite  similar  to  the  addition  product  with  oxygen, 
having  many  properties  in  common,  such  as  solubility,  etc. 


THE  THEORY  OF  VULCANIZATION  63 

but  that  the  rate  of  decomposition,  or  deterioration,  shall  be  not 
greater  than  a  certain  set  figure. 

To  summarize  the  situation  from  the  analyst's  point  of  view: 
vulcanization  is  the  chemical  combination  of  rubber  with  other 
substances,  without  reference  to  time,  temperature,  catalysts 
(except  as  these  remain  as  constituent  parts  of  the  mixture),  or  to 
any  of  the  steps  through  which  the  products  may  have  passed  in 
reaching  the  final  form  in  which  the  rubber  is  found.  For 
example,  there  should  be  no  chemical  difference  between  rubber 
and  sulfur  which  has  combined  a»  such  and  which  has  combined 
by  reason  of  the  treatment  by  Peachey's  process. 


Chapter  V. 
Sampling. 

The  sampling  of  rubber,  and  the  materials  to  be  used  in  the 
manufacture  of  rubber  compounds,  as  is  the  case  with  a  great 
many  other  commercial  and  natural  materials,  is  usually  done  in 
the  most  casual  fashion,  whereas  the  proper  sampling,  and  the 
care  of  the  sample  until  the  analysis  has  been  completed,  is  funda- 
mental. Unless  the  proper  precautions  are  taken  to  make  the 
sample  represent  the  material  from  which  it  was  taken,  and 
maintain  its  condition  and  purity,  not  only  is  the  accuracy  of  the 
analysis  affected,  but  the  incorrect  results  may  frequently  lead  to 
false  conclusions  as  to  the  manufacture  or  composition  of  the 
article.  Samples  for  analysis  have  been  packed,  without 
adequate  protection,  in  the  same  package  with  cans  of  oil; 
ground  rubber  samples  in  unsealed  paper  envelopes  with  bits  of 
excelsior  distributed  throughout;  inner  tube  samples  which  have 
been  light  checked ;  rubber  articles  with  unmistakable  evidence  of 
having  been  placed  against  steam  radiators;  these  do  not  appeal 
to  the  analyst  as  fertile  iields  for  valuable  results.  Samples  of 
less  than  1  gr.  may  be  very  flattering  to  the  ingenuity  and 
ability  of  the  receiver,  but  they  can  hardly  be  said  to  be  repre- 
sentative of  lots  of  finished  goods  weighing  hundreds,  or  even 
thousands  of  pounds. 

The  process  of  sampling  may  be  divided  into  three  stages:  (a) 
the  taking  of  the  sample;  (b)  its  removal  to  the  laboratory;  (c) 
the  preparation  of  the  sample  for  analysis.  The  purpose  of  these 
three  steps  is  to  have  the  actual  material  used  in  making  the 
various  determinations  of  the  same  chemical  composition  as  the 
lot  which  it  represents.  If  the  sample  is  to  represent  a  number  of 
pieces,  the  sample  should  be  drawn  to  represent  a  fair  average 
composition  of  the  lot.  More  often  it  is  not  advisable  to  take 
more  than  one  piece  of  a  lot,  or  even  a  part  of  that.  Under  such 
conditions,  we  cannot  speak  of  average  composition,  but  since  the 
supposition  is  that  the  entire  lot  is  uniform,  and  that  any  one 
piece  (or  part  of  it),  will  truly  represent,  not  the  average,  but 
the  exact  composition  of  all  of  the  rest,  in  such  cases  we  must 

64 


SAMPLING  65 

select  our  samples  at  random.  Speaking  generally,  when  we 
sample  raw  materials  we  should  draw  more  than  one  sample, 
since  these  raw  materials  are  apt  to  vary  throughout  the  lot, 
and  also  because  raw  materials  are  thoroughly  mixed  in  the  proc- 
ess of  manufacture,  and  it  is  the  average  composition  which  is 
of  chief  interest.  With  finished  materials,  the  averaging  process 
has  already  taken  place,  and  it  is  a  fair  risk  to  assume  that  the 
lot  is  uniform.1 

A.  Taking  the  Sample.    If  the  material  is  liquid,  it  should  be 
thoroughly  stirred  before  drawing  off  the  sample.     Particular 
attention  should  be  taken  to  note  whether  there  are  two  liquid 
layers,  or  whether  there  is  any  suspended  matter  (such  as  water 
in  gasoline,  or  foots  in  vegetable  oils).     The  liquid  should  be 
bottled  at  once,  and  sealed  with  a  stopper  which  is  not  attacked 
by  the  liquid.     The  bottle  should  be  scrupulously  clean,  both 
inside  and  outside,  and  should  be  dry.2     Greases,  waxes,  and 
resins,  are  usually  packed  in  small  containers ;  a  few  ounces  may 
be  drawn  from  each  container,  or  from  a  certain  proportion  of 
them.     These  small  samples  are  united  to  form  a  composite 
sample,  which  is  mixed  and  quartered  until  a  final  sample  of 
about  100  to  200  gr.  is  obtained.    This  should  be  placed  in  a 
wide-mouthed  bottle,  or  a  can,  and  sealed. 

Dry  pigments  are  usually  received  in  kegs  or  sacks.  As  in  the 
case  of  greases,  a  small  portion  is  withdrawn  from  some  propor- 
tion of  the  containers ;  these  are  united,  mixed  and  quartered,  and 
a  final  sample  of  100  to  200  gr.  bottled  and  sealed. 

The  sampling  of  crude  rubber  is  discussed  in  connection  with 
the  testing  of  crude  rubber.3 

B.  Transportation  to  the  Laboratory.    The  distance  between 
the  place  where  the  sampling  occurs,  and  that  where  it  is  to  be 
tested,  may  be  a  matter  of  only  a  few  feety  or  it  may  be  hundreds 
of  miles.    The  principles  involved  are  the  same,  irrespective  of  the 

1  This  is   particularly   true  in   rubber  goods,   so  far  as  actual  composition   is 
concerned  ;  but  such  a  sample  will  not  reveal  any  variation  in  the  vulcanization, 
since  the  latter  process  takes  place  in  a  relatively  small  number  of  units.     We 
have  met  cases  of  rubber  belting,  for  instance,   which  is  vulcanized   a   portion 
at  a  time,   where  the  manufacturer  paid  particular  attention  to  the  first  and 
last  part  of  the  belt  because  that  was  where  the  samples  would  be  taken.     No 
amount  of  testing  is  proof  against  such  chicanery. 

2  Samples  of  oil  have  been  received,  the  container  being  an  ink  bottle  in  which 
a  few  drops  of  ink  were  still  to  be  seen  at  the  bottom  of  the  bottle  ;  and  this 
sample  was  to  be  tested  for  mineral  acids ! 

8  Cf .  page  22. 


(if,  THE  ANALYSIS  OF  RUBBER 

distance.  The  containers  in  which  the  raw  materials  are  sent 
.-hnuld  be  tightly  stoppered,  so  as  to  avoid  the  introduction  of 
dirt  and  other  foreign  matter,  and  also  to  prevent  change  in 
composition  through  evaporation  or  leakage.4  Manufactured 
articles  sent  for  analysis  should  be  carefully  wrapped. 

The  principal  deteriorating  agents  of  vulcanized  rubber  are 
heat,  light,  and  oils.  It  is  quite  essential  to  see  that  each  package 
is  not  only  carefully  wrapped,  but  that  it  will  not  come  in  contact 
with  oils,  and  on  packages  which  are  to  be  sent  any  distance. 
-pccific  instructions  should  be  written  on  the  outside,  that  such 
packages  are  to  be  kept  in  a  clean,  cool,  dark  and  dry  place. 
'These  same  precautions  should  be  observed  in  the  laboratory, 
after  the  samples  have  been  received. 

If  considerable  stress  has  been  laid  on  the  care  requisite  for 
delivering  the  sample  to  the  laboratory,  our  justification  is  that 
the  analyst  can  test  only  what  he  receives;  he  cannot  tell  how 
great  a  change,  or  even  at  times  that  any  change  at  all,  has  taken 
place.  Questionable  samples  should  be  discarded  at  once;  failure 
to  do  so  will  often  lead  to  disagreeable  controversies,  which  ac- 
complish no  good  purpose,  and  tend  to  diminish  that  respect 
which  the  analyses  of  the  laboratory  should  inspire. 

C.     Preparation  of  the  Sa?nples  for  Analysis. 

Raw  Materials.  Pigments,  oils,  waxes,  etc.,  should  be  mixed 
thoroughly  before  each  portion  is  taken  for  analysis. 

Unvulcanized  Rubber  Compounds.  Sheet  out  rapidly  on  a  cool 
mill,  roll  between  holland  and  place  in  a  covered  can. 

It(  rldhtud  Rubber.  Treat  as  under  unvulcanized  rubber  com- 
p<  iimds. 

('cmrnt*.  Cements  should  be  stirred  thoroughly  before  por- 
tinns  are  taken  for  analysis,  and  then  immediately  covered.  A 
lair  si/ed  quantity  should  be  taken,  the  solvent  removed,  prefer- 
ably by  evaporation  in  thin  layers  at  room  temperature  (if  ncces- 
s.Mi'y.  to  remove  the  la>t  traces  of  the  solvent,  the  rubber  may  be 
heated  for  a  short  time  between  80  and  90C,  but  it  is  better  to 
avoid  heating  of  any  kind),  and  the  residue  sheeted  out  and 
foiled  between  holland.  as  in  the  ease  of  other  unvulcanized 
'•(  unnounds. 


•  sent    to   the    laboratory   in    a    can    without  a   cover, 
in?    hours,    ami    was    not    discovered    until    the    next 

•  ration   had    taken   place,   and   the  residue  was  hardly 


SAMPLING  67 

Vulcanized  Rubber  Samples.  Strip  off  all  fabric,  and  see  that  the 
rubber  is  homogeneous,  i.e.,  that  there  are  not  two  or  more  com- 
pounds in  the  sample. 

Grind  about  50  gr.  in  a  meat  grinder,  or  coffee  grinder,  or 
by  passing  between  the  tightly  closed  rolls  of  a  laboratory  mill. 
Sift  the  ground  material  through  a  20  mesh  screen  until  about 
25  gr.  has  been  collected.  It  is  not  necessary  to  sift  the  entire 
amount  of  50  gr. 

The  type  of  grinder  is  immaterial,  providing  the  following  pre- 
cautions are  observed:  The  sample  must  be  ground  at  room  tem- 
perature, without  being  appreciably  heated  up;  no  metal  must 
be  introduced  into  the  sample  during  the  grinding;  and  prefer- 
ence should  be  given  to  those  grinders  which  tear  the  sample 
rather  than  just  cut  it  up,  since  the  former  gives  the  greater  sur- 
face for  extraction. 

Material  containing  fabric  and  rubber  in  such  a  manner  as  to 
make  it  impossible  to  produce  good  separation,  shall  be  cut  with 
scissors  into  as  small  pieces  as  is  practicable.  Rubber  and  fabric 
cannot  be  ground  together,  since  segregation  will  be  certain  to 
occur  on  account  of  the  difference  in  behavior  on  grinding,  and 
this  holds  true  even  if  the  entire  sample  is  ground  and  sifted. 

Hard  rubber  samples  are  prepared  for  analysis  by  rasping. 

Insulated  wire  should  be  cleaned  with  a  damp  cloth,  to  remove 
any  adhering  cotton  or  other  adhering  material,  but  care  must  be 
exerted  to  see  that  waxy  hydrocarbons  are  not  removed  from  the 
surface.  If,  however,  a  saturated  braid  sample  must  be  used, 
remove  the  braid,  and  sandpaper  the  insulation  for  a  depth  of  at 
least  .005  in.,  and  wipe  with  a  damp  cloth.  This  treatment  will 
probably-  give  low  results  for  waxy  hydrocarbons,  and  hence 
should  be  resorted  to  only  when  absolutely  necessary,  and  a 
statement  regarding  the  treatment  given  the  sample  should  al- 
ways be  included  as  a  part  of  the  report  of  the  analysis.  On  the 
other  hand,  it  should  always  be  indicated  when  analyses  are  made 
on  samples  which  have  been  braided,  or  which  have  been  vul- 
canized in  contact  with  a  saturated  braid  or  tape,  since  there  will 
be  a  migration  of  the  liquid  hydrocarbons  of  the  saturation  from 
the  braid  or  tape,  into  the  rubber  insulation,  and  although  the 
waxy  hydrocarbons  may  be  a  bit  low,  because  of  the  sandpaper- 
ing of  the  surface,  the  acetone  extract  and  the  liquid  hydrocar- 
bons will  be  high. 


Chapter  VI. 
Extractions. 

Certain  organic  substances,  mainly  the  oils  and  waxes,  are 
removed  by  extract  with  acetone,  chloroform,  or  by  saponification 
with  alcoholic  potash.  The  results  obtained  by  these  three  opera- 
tions are  largely  qualitative,  and  from  them  may  be  obtained  a 
fair  index  as  to  the  quality  of  the  article  as  a  whole.  In  addition, 
there  are  some  substances  which  may  be  determined  quite  accu- 
rately in  these  extracts. 

Extraction  Apparatus.  The  extraction  apparatus  should  com- 
ply with  the  following  requirements:  It  should  be  of  the  reflux 
type,  with  the  condenser  placed  immediately  above  the  cup  which 
holds  the  sample;  the  sample  must  be  suspended  in  the  vapor  of 
the  boiling  solvent;  the  cup  must  be  of  the  syphon  type;  the  cup 
must  be  far  enough  away  from  the  sides  of  the  extraction  flask 
that  it  will  be  maintained  at  the  temperature  of  the  boiling  point 
of  the  solvent;  only  glass  or  metal  joints  may  be  used — there 
shall  be  no  cork,  rubber,  or  similar  material  in  the  extractor,  with 
which  the  solvent  may  come  in  contact,  and  from  which  extract- 
able  matter  may  be  obtained. 

The  extraction  flask  may  be  of  a  size  that  will  permit  it  to  be 
weighed  directly,  or  it  is  permissible  to  transfer  the  extract  to  a 
smaller  flask  for  evaporation,  drying,  and  weighing.  The  Cottle 
(better  known  as  the  Underwriters),  the  Joint  Rubber  Insula- 
tion Committee,  and  Bureau  of  Standards  types  are  all  satisfac- 
tory, and  may  be  relied  upon  to  give  equally  accurate  results,  but 
any  extractor  which  fulfills  the  above  requirements  will  do. 

Acetone  Extract. 

The  acetone  used  in  this  extraction  must  be  redistilled,  and 
free  from  water  or  acid.  It  should  be  distilled  over  sodium  or 
potassium  carbonate,  and  kept  in  clean  dark-colored  glass  bottles. 

Place  2.000  gr.  of  the  sample  in  an  acetone  extracted  paper 

68 


EXTRACTIONS  69 

thimble,  or  fold  it  in  an  extracted  filter  paper;  insert  in  the 
syphon  cup,  and  extract  continuously  for  eight  hours.1  The  heat- 
ing must  be  controlled  so  that  the  solvent  syphons  about  20  times 
per  hour.  Remove  the  solvent,  dry  the  flask  and  contents  at  900 
to  constant  weight,2  and  calculate  to  percentage.  This  figure  is 
usually  called  the  "acetone  extract,  unconnected."  Due  record 
should  be  made  of  the  color  and  odor  of  the  extract,  and  of  any 
other  peculiarities  which  may  be  noticeable.  With  high  free 
sulfur,  or  waxy  hydrocarbons,  these  substances  will  separate  out 
on  the  sides  of  the  flask. 

Reserve  the  residue  for  the  chloroform  extraction. 

The  acetone  dissolves  the  unchanged  or  free  sulfur,  vegetable 
fats  or  oils,  rosin,  mineral  oils,  paraffin,  ceresin,  ozokerite,  a  con- 
siderable portion  of  bituminous  substances  such  as  the  mineral 
rubbers,  tars,  etc.,  and  the  so-called  resins  of  the  crude  rubber. 
In  simple  mixtures,  the  separate  constituents  may  be  determined 

1  Eight  hours  should  suffice  for  any  properly  prepared  sample  extracted  under 
exact  conditions.     However,  some  uncured  samples  may  fuse  together  into  a  solid 
mass,  and  require  a  longer  time  for  comparatively  complete  extraction.    In  such 
cases,  extract  until  the  solution  in  the  extraction  cup  is  colorless,  and  continue 
for  four   hours   longer.     Uncured  samples   should   be   sheeted   thin  and    rolled 
between  hardened  filter  paper,  to  effect  a  thorough  and  more  rapid  extraction. 
The  expression  "complete  extraction"  is  a  misnomer :   the  free  sulfur  actually 
is  extracted  in  the  first  four  hours,  but  the  soluble  organic  matter  is  extracted 
with  difficulty.    Additional  quantities  of  extract  can  be  dissolved  up  to  48  hours, 
or  even  more,  but  the  amount  so  obtained  is  but  a  small  proportion  of  the  total, 
and  is  more  or  less  constant    Hence,  if  we  interrupt  the  extraction  at  a  definite 
point,  we  secure  results  which    serve  the  purpose  of  indicating  the  quality  of 
the  rubber,  and  are  comparable  with  other  extracts  made  in  a  similar  manner. 
The  same  is  true  to  a  large  extent  with  the  chloroform  and  alcoholic  potash 
extractions,    and   we   really  deal   with    comparable,    rather   than    with   absolute 
values. 

The  necessity  for  continuous  extraction  is  explained  on  the  same  basis ;  with 
samples  of  approximately  the  same  degree  of  fineness,  the  extraction  is  a  matter 
of  time,  rather  than  the  number  of  times  the  syphon  empties;  hence,  standing 
overnight  would  permit  the  solvent  to  extract  a  considerable  quantity  of  soluble 
matter  that  would  not  otherwise  be  extracted.  Many  of  the  variations  in  check 
results  are  really  due  to  faulty  manipulation,  rather  than  to  the  type  of  extrac- 
tor, or  fineness  of  the  sample. 

2  There  has  been   considerable  discussion  as  to   the  adoption   of   a   standard 
time  for  drying.     Some  samples  are  dry  in  half  an  hour  and  it  is  a  waste  of 
time  to  continue  for  hours  longer.     On  the  other  hand,  the  Joint  Rubber  Insula- 
tion Committee  found  some  samples,  notably  those  high  in  solid  hydrocarbons, 
which  were  not  dry  in  two  hours.     Sometimes  in   the  hot,  humid  months  of 
summer,  water  may  condense  on  the  outside  of  the  condenser  of  the  extractor, 
and  some  of  this  may  find  its  way  into  the  extraction  flask.    If  it  does,  it  must 
be  removed,  even  if  it  does  take  more  than  half  an  hour ;  it  is  not  extract,  and 
must  not  be  weighed  as  such.     Of  course,  the  longer  periods  for  drying  may 
lose  a  little  more  of  the  free  sulfur  than  the  shorter  periods ;  especial  care 
must  be  taken  to  see  that  the  temperature  does  not  go  over  90C,  for  even  at 
this  temperature,  there  is  some  loss  of  free  sulfur  and  at  higher  temperatures, 
over  an  extended  period,  the  loss  may  be  very  great. 


70  THE  AXALYSIt  OF  RUBBER 

with  sonic  accuracy,  but  in  the  more  complex  ones  only  a  few 
of  the  constituent-  may  be  determined  with  sufficient  accuracy  to 
be  of  any  value.  The  1'rce  sulfur  may  always  be  determined  with 
a  high  degree  of  accuracy;  in  the  absence  of  tars  and  mineral 
rubber,  parailin  and  ceresin  are  capable  of  being  determined  with 
equal  accuracy.  Fatty  oils  will  be  associated  with  the  rubber 
resins,  and  if  we  assume  that  the  latter  are  about  3.5  to  rl%  of 
the  rubber  present,  we  may  get  a  fair  line  on  the  quantity  of 
vegetable  oils,  but  such  a  scheme  is  only  approximate. 

lvd>in  may  be  determined  by  the  method  of  K.  ,1.  Parry.11 
The  fatty  acid<  are  dissolved  in  20  cc.  of  95r/o  alcohol,  a  drop  of 
phcnolphthalein  is  added,  and  then  strong  caustic  soda  (one  part, 
of  alkali  to  two  parts  of  water)  until  the  reaction  is  just  alkaline. 
The  solution  is  heated  for  a  few  minutes,  allowed  to  cool,  and 
then  transferred  to  a  100  cc.  stoppered  graduated  cylinder.  The 
latter  is  filled  to  the  mark  with  ether,  2  gr.  of  powdered  silver 
nitrate  is  added,  and  the  mixture  shaken  vigorously  for  fifteen 
minutes,  in  order  to  convert  the  acids  into  their  silver  salts. 
When  the  insoluble  salts  have  settled,  50  cc.  of  the  clear  solution 
(containing  the  silver  salts  of  rosin)  is  pipetted  off  into  a  second 
100  cc.  cylinder,  and  shaken  with  20  cc.  dilute  hydrochloric  acid 
il  acid  to  2  water).  The  ethereal  layer  is  drawn  off,  and  the 
aqueous  layer  is  shaken  twice  with  ether.  The  ether  extracts  are 
united,  washed  with  water,  and  the  ether  distilled  off  in  a 
weighed  beaker.  The  residue,  rosin,  is  dried  at  110  to  115C, 
cooled,  and  weighed. 

This  is  an  excellent  means  of  separating  fatty  oils  and  rosin; 
it  is  best  performed  by  taking  the  water  solution  in  the  deter- 
mination of  unsaponifiable  matter,  making  it  acid  with  hydro- 
chloric acid,  and  extracting  the  liberated  fatty  acids  with  ether. 
The  ether  must  be  driven  off,  and  the  fatty  acids  dried,  before 
the  method  may  be  used. 

The  mineral  oils  can  be  partly  separated  from  hard  paraffin, 
sufficiently  so  as  to  give  some  indication  of  the  composition.  So 
far  as  our  experience  goes,  no  method  has  been  given  which  will 
determine  the  relative  amounts  of  mineral  rubber  and  paraffin 
in  a  mixture  of  the  two.  The  possibilities  of  such  a  method 
being  developed  arc  very  remote,  in  view  of  the  wide  variations  in 

3.  Mien's  ('(iiiiincj'fiiil   Or.n.-iim.-   A  iinlysis.   -lib   cd..    Vol.    V,   p.   7.1;. 


EXTRACTIONS  71 

the  composition  of  the  mineral  rubbers,  and  the  fact  that  chemi- 
cally they  are  so  nearly  like  paraffin. 

Chloroform  Extract. 

The  chloroform  extraction  is  performed  in  the  same  apparatus 
used  in  making  the  acetone  extraction.  The  chloroform  should 
be  redistilled  over  alkali. 

Extract  for  four  hours,  the  residue  from  the  acetone  extraction 
(it  is  not  necessary  to  remove  the  acetone  adhering  to  the 
sample) ,  using  about  60  cc.  of  the  solvent.  If  at  the  end  of  four 
hours,  the  solvent  in  the  syphon  cup  is  still  colored,  continue  to 
extract  until  it  is  colorless.  Filter  the  extract  through  fat  free 
filter  paper  into  a  small  Erlenmeyer  flask,  distil  off  the  solvent, 
and  dry  the  flask  and  contents  to  constant  weight  at  95C. 

If  the  chloroform  extraction  cannot  be  started  immediately 
after  the  acetone  extraction  has  been  completed,  the  sample 
should  be  protected  against  oxidation  by  keeping  it  in  a  vacuum 
desiccator  in  a  vacuum  of  at  least  50  mm  of  mercury.  Vulcanized 
rubber  which  has  been  extracted  with  acetone  oxidizes  very 
rapidly  in  the  air  and  the  resultant  products  are  so  soluble  in 
chloroform  as  to  yield  hopelessly  false  results,  being  as  much  as 
five  to  ten  times  the  true  amount. 

Reserve  the  residue  from  the  chloroform  extraction  for  treat- 
ment with  alcoholic  potash. 

The  chloroform  dissolves  part  of  the  rubber,  particularly  the 
undercured,  and  the  oxidized  rubber.  Its  chief  value  is  that  it 
dissolves  part  of  the  mineral  rtfbber,  the  solution  taking  on  an 
intense  brown  or  black  color.  It  is  an  invaluable  qualitative  test 
for  mineral  rubbers,  the  color  being  quite  distinctive,  and  not 
likely  to  be  mistaken  for  anything  else. 

The  chloroform  extract  in  a  well  cured  and  unoxidized  sample 
of  soft  vulcanized  rubber,  will  run  from  1  to  3%  of  the  rubber 
present,  with  the  average  nearer  the  lower  figure.  It  has  been 
suggested  as  means  of  determining  whether  or  not  the  rubber  has 
been  undercured,  but  the  data  available  are  largely  limited  to 
insulation  compounds,  and  are  not  entirely  convincing. 

Alcoholic  Potash  Extract. 

Dry  the  residue  from  the  chloroform  extraction  at  60C  until 
the  odor  of  chloroform  is  no  longer  noticeable.  Place  the  rubber 


72  THE  ANALYSIS  OF  RUBBER 

in  a  200  cc.  Erlenmeyer  flask,  and  cover  with  50  cc.  normal  alco- 
holic potash.4  Boil  for  four  hours  under  reflux  condenser.  Filter 
by  decantation  through  a  hardened  filter  paper,  wash  with  two 
portions  of  25  cc.  of  hot  alcohol,  and  then  thoroughly  with  hot 
water.  Evaporate  the  filtrate  to  dryness,  take  up  in  warm  water 
and  when  the  solution  has  been  effected,  cool  to  room  tempera- 
ture. Transfer  to  a  separatory  funnel,  add  30  cc.  N/5  hydro- 
chloric acid  and  sufficient  water  to  bring  the  total  up  to  about 
100  cc.  Add  40  cc.  of  ethyl  ether,  shake  thoroughly,  allow  to 
stand  until  the  two  layers  are  completely  separated,  draw  off  the 
water  into  a  second  separatory  funnel,  and  continue  to  extract 
with  20  cc.  portions  of  ether  until  a  colorless  solution  results,  and 
then  twice  more.  Unite  all  the  ether  fractions  in  the  first  separa- 
tory funnel,  and  wash  with  water  until  the  water  shows  no 
further  acidity  (test  with  silver  nitrate  solution).  Filter  the 
ether  through  a  plug  of  extracted  cotton  into  a  weighed  beaker  or 
flask,  evaporate  to  dryness,  and  dry  to  constant  weight  at  95C. 
Another  method  for  the  determination  of  the  alcoholic  potash 
extract  is  to  dry  the  residue  from  the  chloroform  extract,  cool, 
and  weigh.  Place  the  rubber  residue  in  a  200  cc.  Erlenmeyer 
flask,  add  50  cc.  N/l  alcoholic  potash,  and  boil  under  a  reflux 
condenser  for  four  hours.  Filter  off  the  rubber  on  a  Gooch  or 
alundum  crucible,  wash  with  hot  alcohol,  and  then  hot  water,  un- 
til the  washings  are  free  from  alkali;  dry  in  an  inert  atmosphere 
to  constant  weight;  the  loss  in  weight  is  the  oil  substitute.5 

*  We  should  not  be  led  astray  by  those  who  wish  to  replace  potassium 
hydroxide  with  sodium  hydroxide.  When  the  former  was  difficult  to  obtain,  one 
did  what  could  be  done  with  the  material  which  was  available,  but  no  question 
of  a  slightly  higher  cost  should  interfere  now  with  the  use  of  a  better  and 
more  widely  known  reagent.  On  the  other  hand,  with  pure  grain  alcohol  diffi- 
cult to  obtain  under  present  conditions  in  the  United  States,  the  use  of 
methylated  alcohol  becomes  almost  obligatory.  It  is  hard  to  see  just  what  error 
would  be  introduced  by  the  presence  of  methyl  alcohol ;  it  is  difficult  to  con- 
ceive of  anything  which  might  be  present  in  a  rubber  compound  which  is  soluble 
in  methyl  alcohol,  and  insoluble  in  acetone,  chloroform  or  ethyl  alcohol.  If 
the  analyst  will  see  that  his  denatured  alcohol  has  been  denatured  with  methyl 
alcohol,  and  will  use  this  denatured  alcohol  only  after  redistillation  over  caustic 
potash,  the  chances  for  error  are  very  small  indeed.  Of  course,  in  any  event, 
and  regardless  of  the  kind  of  alcohol  used,  a  blank  is  always  run,  and  due  cor- 
rection made  for  the  results  found.  Again,  no  careful  analyst  will  use  an  alco- 
holic potash  solution  which  has  been  standing  a  long  time,  and  particularly  if 
it  is  badly  discolored. 

8  This  method  is  not  as  accurate  as  the  previously  mentioned  one,  and  is  not 
to  be  recommended.  There  is  the  greatest  difficulty  in  washing  out  all  of  the 
alkali,  and  the  latter  cannot  be  removed  with  acids  on  account  of  the  proba- 
bility of  these  acids  attacking  some  of  the  pigments  in  the  rubber. 


EXTRACTIONS  73 

Ordinary  crude  rubber  shows  a  small  amount  of  material  solu- 
ble in  alcoholic  potash,  usually  around  1%  of  the  amount  of 
rubber.  This  will  be  included  in  the  results  in  either  method. 
In  the  first  method,  we  weigh  the  fatty  acids,  although  they  were 
present  originally  as  the  glycerides;  i.e.,  we  weigh  only  95%  of 
the  substitute.  These  two  elements  tend  to  neutralize  each  other, 
and  the  result  is  a  pretty  accurate  determination  of  the  amount  of 
fatty  substitute,  not  including,  of  course,  any  unchanged  oil  which 
would  have  been  extracted  in  acetone,  or  any  pigments  contained 
in  the  substitute. 

If  vegetable  oils  are  used,  and  there  is  sufficient  sulfur  present, 
we  may  find  that  a  part  of  the  oil  has  been  converted  into  an 
insoluble  form,  and  will  appear  at  this  point.  There  is  no  way 
to  distinguish  substitute  formed  during  vulcanization  and  that 
added  as  such,  excepting  that  the  oil  in  the  substitute  is  usually 
very  well  changed  into  the  acetone-insoluble  form,  whereas  the 
oil  added  as  such  will  be  changed  to  only  a  slight  extent. 

The  method  involving  loss  of  weight  is  practically  worthless, 
because  it  is  an  almost  hopeless  task  to  thoroughly  wash  out  the 
alkali.  In  one  case  continuous  washing  for  8  hours  did  not  suf- 
fice, and  acid  cannot  be  used  to  neutralize  the  alkali,  on  account 
of  its  effect  on  the  acid-soluble  fillers. 

Analysis  of  Acetone  Extract, 

Unsaponifiable  Matter. 

Add  to  the  acetone  extract,  50  cc.  of  alcoholic  potash,  boil  under 
a  reflux  condenser  for  two  hours,  and  evaporate  to  dryness.  Add 
10  cc.  of  water  and  20  cc.  of  ether,  heat  until  solution  is  complete ; 
cool,  and  transfer  to  a  separatory  funnel,  wash  out  with  warm 
water,  and  cool,  then  with  two  20  cc.  portions  of  ether;  the  separa- 
tory funnel  should  contain  100  cc.  of  water,  and  not  less  than  40 
cc.  of  ether.  Shake  vigorously,  allow  the  two  layers  to  separate, 
and  draw  off  the  aqueous  layer  into  a  second  separatory  funnel. 
Repeat  the  extraction  until  no  further  material  can  be  extracted 
(not  less  than  four  extractions  should  be  made) .  Unite  the  ether 
portions  of  the  extract,  and  wash  with  water  until  free  from  alkali 
(the  first  two  portions  may  be  united  with  the  original  aqueous 
solution,  and  the  whole  reserved  for  the  determination  of  free 
sulfur) .  Filter  the  ethereal  layer  through  extracted  cotton,  wash- 


74  THE  ANALYSIS  OF  RUBBER 

ing  with  ether  and  hot  chloroform,  using  the  latter  to  rinse  the 
original  flask,  and  both  separatory  funnels.  Evaporate  to  dry- 
ness,  dry  to  constant  weight  at  95  to  100C,  cool  and  weigh. 

The  above  method  gives  the  liquid  and  solid  hydrocarbons,  and 
the  unsaponifiable  resins.  The  difference  between  the  total  ex- 
tract, and  the  sum  of  the  free  sulfur  and  unsaponifiable  matter, 
will  consist  of  the  saponifiable  resins,  and  any  fatty  oils  which 
may  have  been  extracted.  The  acetone  soluble  matter  of  the 
mineral  rubber  will  be  found  largely  in  the  unsaponifiable  por- 
tion. Rosin,  as  its  composition  indicates,  will  be  distributed  be- 
tween the  two,  about  90%  being  saponifiable. 

Waxy  Hydrocarbons. 

The  time-honored  method  for  separating  the  solid  paraffins  has 
been  to  dissolve  the  unsaponifiable  portion  in  alcohol,  and  freeze 
out  the  paraffin.  However,  some  of  the  latter  will  always  remain 
in  the  alcohol,  along  with  any  liquid  mineral  oils,  and  the  un- 
saponifiable rubber  resins.  The  Joint  Rubber  Insulation  Com- 
mittee devised  a  method  for  correcting  for  the  alcohol  soluble 
paraffin,  in  the  absence  of  mineral  oils,  or,  if  the  latter  were 
present,  to  get  the  total  soluble  paraffins  and  the  mineral  oil 
together.  The  alcohol  insoluble  paraffins  are  called  "Waxy  hydro- 
carbons A"  and  the  soluble  paraffins  are  called  "Waxy  hydrocar- 
bons B."  If  the  latter  are  solid,  the  sum  of  the  two  is  the  total 
paraffin  in  the  sample. 

If  it  is  desired  to  know  only  the  total  mineral  hydrocarbons, 
then  the  method  for  Waxy  hydrocarbons  B  is  used  directly. 

Waxy  Hydrocarbons  A. 

Add  50  cc.  absolute  alcohol  to  the  unsaponifiable  matter  and 
warm  until  solution  is  as  complete  as  possible.  Cool  the  solution 
to  —  4  or  —  5C,  and  maintain  at  this  temperature,  or  lower,  by 
packing  the  flask  in  a  mixture  of  ice  and  salt.  Filter  out  the 
waxy  hydrocarbons,  using  a  funnel  packed  with  ice  and  salt  and 
applying  suction  if  necessary.  Wash  the  flask  and  filter  with 
25  cc.  of  95%  alcohol  which  has  been  previously  cooled  to  the 
same  temperature.  Dissolve  the  residue  on  the  filter  paper  in 
hot  chloroform  into  the  original  flask;  evaporate  the  chloroform, 
and  dry  the  residue  at  95  to  100C  to  constant  weight. 


EXTRACTIONS  75 


Waxy  Hydrocarbons  B. 

Evaporate  the  alcohol  from  the  determination  of  Waxy  hydro- 
carbons A,  add  25  cc.  of  carbon  tetrachloride,  and  transfer  to  a 
separatory  funnel.  Shake  with  cone,  sulfuric  acid,  drain  off  the 
discolored  acid,  and  repeat  with  fresh  portions  of  the  acid  until 
there  is  no  longer  any  discoloration.  Vigorous  shaking  is  abso- 
lutely necessary  for  the  success  of  the  method.  After  drawing 
off  all  of  the  acid,  wash  the  carbon  tetrachloride  solution  with 
repeated  portions  of  water  until  all  traces  of  acid  are  removed.6 
Transfer  the  carbon  tetrachloride  solution  to  a  weighed  flask, 
evaporate  off  the  solvent,  and  dry  to  constant  weight  at  95  to 
100C.  Note  whether  the  residue  is  solid,  liquid,  or  pasty. 

9  On  account  of  the  specific  gravity  of  the  carbon  tetrachloride  washing  with 
water  is  a  very  tedious  proposition,  because  the  carbon  tetrachloride  must  be 
drawn  off  with  each  washing,  and  returned  to  the  flask.  While  the  Joint  Rubber 
Insulation  Committee  did  not  recommend  it,  the  carbon  tetrachloride  may  be 
diluted  with  ether  until  the  mixed  solvents  have  a  gravity  lower  than  that  of 
water ;  the  washing  can  then  be  continued  as  usual  with  separatory  funnel 
washings.  Ether  to  the  extent  of  about  two  and  a  half  to  three  times  the 
volume  of  carbon  tetrachloride  will  be  necessary  to  have  the  ether-tetrachloride 
mixture  float  on  the  water  layer. 


Chapter  VII. 
The  Determination  of  Rubber. 

It  is  a  peculiar  fact  concerning  the  analysis  of  rubber  that  the 
determination  of  the  principal  constituent  involved  is  seldom,  if 
ever,  made  by  a  direct  determination.  A  tremendous  amount  of 
research  has  been  undertaken,  methods  and  revisions  of  methods 
have  been  suggested,  but  as  yet  no  one  method  has  succeeded  in 
securing  the  endorsement  of  the  rubber  analysts. 

The  methods  for  the  determination  of  rubber  may  be  classified 
under  three  headings:  (1)  direct;  (2)  indirect;  (3)  difference. 
In  No.  1,  the  idea  is  to  form  a  definite  compound  with  rubber 
and  either  weigh  the  compound  directly  or  determine  some  part 
of  the  compound  and  from  these  figures  to  calculate  the  total 
rubber.  The  two  principal  methods  in  this  group  are  the  tetra- 
bromide  methods  and  the  nitrosite  methods.  The  indirect 
methods  (No.  2),  proceed  to  separate  the  rubber,  but  the  latter 
i<  not  determined  as  such,  but  is  determined  as  the  loss  during 
the  solution.  The  difference  methods  comprise  the  third  group, 
and  the  principle  involved  is  merely  to  determine  every  other 
known  constituent,  subtract  the  total  from  100%,  and  call  the 
remainder  rubber. 

The  Tetrabromide  Method. 

The  tetrabromide  method  was  first  advocated  by  Budde,  for 
use  in  determining  the  rubber  in  unvulcanized  compounds,  or 
crude  rubber.  The  bromination  solution  used  was  6  gr.  of  bromide 
and  1  gr.  of  iodine  in  1000  cc.  of  carbon  tetrachloride.  The  rubber 
was  swollen  in  carbon  tetrachloride,  and  filtered.  The  clear  solu- 
tion was  treated  with  50  cc.  of  the  bromine  solution,  allowed  to 
stand  for  24  hours,  diluted  with  an  equal  volume  of  alcohol,  and 
when  the  precipitate  had  settled  it  was  filtered  and  washed  with 
carbon  tetrachloride-alcohol  (1-1),  and  finally,  to  remove  the  bro- 
mine, with  alcohol.  The  precipitate  was  weighed  as  C10H16Br4, 
and  calculated  to  rubber,  using  the  factor  0.298. 

76 


THE  DETERMINATION  OF  RUBBER  77 

The  gravimetric  method  did  not  prove  successful,  and  evoked 
considerable  criticism.  Fendler,  Harries,  Hubener,  Spence,  and 
others,  presented  various  modifications,  and,  in  the  meanwhile, 
Budde  had  published  a  volumetric  method,  which  he  claimed  was 
satisfactory  for  rubber  vulcanized  with  sulfur  chloride.  In  this 
method,  the  rubber  swollen  in  carbon  tetrachloride  was  treated 
with  the  brominating  mixture,  and  after  the  tetrabromide  had 
been  filtered  free  from  bromine,  it  was  treated  with  cone,  nitric 
acid  and  N/5  silver  nitrate,  and  the  bromine  determined  as  in 
Volhard's  method. 

The  volumetric  method  did  not  prove  any  more  acceptable  than 
the  gravimetric.  For  some  compounds,  good  results  were  ob- 
tained, but  in  others,  especially  with  vulcanized  rubber,  it  was 
found  that  the  bromine  did  not  replace  the  sulfur  of  vulcaniza- 
tion. There  was  also  found  to  be  a  loss  of  bromine  during  the 
acid  treatment,  which  Spence  corrected  by  fusing  the  tetra- 
bromide with  alkali.  Vulcanized  samples  gave  low  results,  but 
when  it  was  noted  that  the  sulfur  combined  with  the  double  bonds 
of  rubber  in  stoichiometric  proportions,  it  was  seen  that  by  adding 
to  the  bromine  found  in  the  tetrabromide  the  bromine  equivalent 
of  the  sulfur  of  vulcanization  (2Br  =  S),  the  results  were  more 
uniform,  and  more  nearly  correct.  It  was  also  noticed  that 
hydrobromic  acid  was  formed  during  bromination,  and  efforts 
were  made  to  eliminate  this  factor  by  freezing  the  brominating 
solution,  but,  on  the  whole,  while  cooling  reduced  the  formation  of 
hydrobromic  acid,  it  did  not  eliminate  it. 

Recently,  further  attempts  have  been  made  to  make  the  method 
practical.  Lewis  and  McAdam *  published  a  modification  based 
on  Mcllhenny's2  method  for  the  determination  of  substitution, 
and  Fisher,  Gray  and  Merling3  have  recommended  some  im- 
provements in  the  Lewis  and  McAdam  method. 

When  bromine  adds  to  rubber,  whether  the  latter  be  vulcanized 
or  not,  there  are  a  number  of  ways  in  which  the  reaction  may 
progress: 

(1)  HC  :  CH  +  2  Br  =  HCBr.HCBr  //  _  c  '  £" 

(2)  HCBr.HCBr          =  HC  :  CBr  +  HBr 

(3)  HC  :  CBr  +  2  Br  =  HCBr.CBr2 

(4)  HC  :  CH  +  HBr  =  HCBr.CH2 

(5)  CH2.CH2  +  Br2     =  CHBr.CH2  +  HBr 

1  J.  Ind.  Eng.  Chem.  12,  675-6   (1920). 
3J.  Am.  Chem.  Soc.  21,  1084  (1899). 
»J.  Ind.  Eng.  Chem.  13,  1031-4   (1921). 


78  THE  ANALYSIS  OF  RUBBER 

The  first  reaction  is  purely  additive;  the  second  is  a  splitting 
off  of  HBr,  re-forming  the  double  bond,  which  again  combines 
with  2  Br  as  shown  in  No.  3 ;  the  liberated  hydrobromic  acid  may 
unite  at  a  new  double  bond,  as  in  No.  4;  and  finally,  No.  5  is  a 
case  of  straight  substitution.  The  resulting  product  will  contain 
HCBr.HCBr;  HCBr.CH2;  HCBr.CBr2;  HBr  and  Br.  It  is  ap- 
parent that  every  molecule  of  HBr  remaining  uncombined  with 
the  rubber  represents  a  loss  of  2  Br  from  the  excess  over  that 
required  for  the  double  bonds.  This  has  been  one  of  the  serious 
errors,  and  it  is  one  which  varies  greatly  with  variations  in  the 
condition  of  time,  temperature,  and  concentration  of  the  solutions. 

In  determining  the  iodine  number  of  "burnt"  linseed  oils,  Smith 
and  Tuttle  4  found  that  concordant  results  could  be  obtained  only 
when  a  very  exact  procedure  was  followed,  in  which  the  weight 
of  the  sample,  volume  and  strength  of  the  iodine  solution,  time 
and  temperature  of  the  reaction,  were  specified  within  very  nar- 
row tolerances.  The  analogy  in  chemical  reactions,  between  dry- 
ing oils  and  rubber,  is  very  striking,  and  we  may  expect  to  find 
just  as  great  difficulties  with  vulcanized  rubber  as  with  oxidized 
linseed  oil.  Lewis  and  McAdam  brominate  for  2-4  hours,  while 
Fisher,  Gray  and  Merling  say  2.5  to  3.5  hours.  The  amount  of 
the  sample  is  quite  indefinite,  not  over  2.00  gr.  for  unvulcanized ; 
1.50  to  2.00  gr.  for  vulcanized.  In  the  latter  case,  no  special  at- 
tention seems  to  have  been  paid  to  whether  the  material  contained 
30  or  90%  of  rubber  hydrocarbons ;  nor  to  whether  they  are  deal- 
ing with  rubber  having  a  vulcanization  coefficient  of  2.0  or  10.0. 
There  is  a  considerable  amount  of  lack  of  definiteness  in  a  reac- 
tion which,  in  a  similar  determination  (adding  halogen  to  partb 
oxidized  oils),  has  been  shown  to  be  absolutely  essential.  It 
perhaps  not  to  be  wondered  at  that  after  so  many  years  of  trial, 
there  still  remains  such  an  element  of  doubt. 

In  spite  of  the  iact  that  it  requires  an  extra  determination 
(sulfur  of  vulcanization),  the  tetrabromide  method  offers  an 
excellent  opportunity  for  a  direct  method,  if  some  one  will  take 
the  time  to  ascertain  the  exact  conditions  which  are  necessary 
for  consistent  results. 

«J.  Ind.  and  Eng,  Chem,  6,  994   (1914). 


THE  DETERMINATION  OF  RUBBER  79 


Nitrosite  Method. 

The  nitrosite  method  was  first  described  by  Harries.5  He 
found  that  by  allowing  N203  to  react  for  a  sufficient  time  with 
a  solution  of  rubber,  he  obtained  a  final  product  of  constant 
composition,  C10H15N30T,  and  by  weighing  this  material,  the  rub- 
ber hydrocarbon  could  be  calculated.  Rubber  resins  were  re- 
moved by  extraction  with  acetone.  Alexander  in  attempting  to 
repeat  Harries  work,  claimed  that  during  the  formation  of  the 
nitrosite,  carbon  dioxide  was  given  off,  and  that  the  composition 
of  the  final  nitrosite  of  Harries  was  really  C9H12N.,06.  The  loss 
of  carbon  dioxide  has  not  been  confirmed  by  any  other  workers  in 
this  field,  so  that  if  carbon  dioxide  was  really  lost,  as  Alexander 
says,  it  was  because  of  some  condition  not  in  accordance  with 
Harries  method.  However  that  may  be,  every  one  agreed  upon 
the  difficulty  in  getting  a  constant  composition  for  their  end 
product,  and  this  method  was  more  or  less  abandoned  until  Wes- 
son 6  took  it  up  from  a  new  angle.  He  proved  that  Alexander 
was  wrong  in  his  statement  that  carbon  dioxide  was  lost,  and 
hence,  although  the  final  product  varied  in  composition,  it  still 
held  all  of  its  original  carbon.  If,  therefore,  the  carbon  in  the 
nitrosite  was  determined,  a  simple  calculation  could  be  made 
directly  to  rubber  which  did  not  depend  upon  any  definite  com- 
position of  the  nitrosite  and  was  independent  of  the  sulfur  of 
vulcanization.  Wesson  secured  some  good  results  on  a  few  com- 
pounds, but  with  some  complicated  commercial  compounds,  large 
differences  cropped  out.  Tuttle  and  Yurow,7  while  investigating 
the  possibilities  of  Wesson's  method,  found  that  his  best  results 
were  obtained  by  a  fortunate  balancing  of  errors,  and  that  when 
the  causes  for  these  errors  were  removed,  accurate  results  could 
be  obtained  directly  in  the  presence  of  practically  any  known 
organic  or  inorganic  fillers.  The  only  unfortunate  circumstance 
connected  with  this  method  is  the  fact  that  it  requires  a  fairly 
complicated  combustion  train,  and  this  has  prevented  many 
laboratories  from  testing  it  out.  Perhaps  further  experimentation 
along  the  lines  of  wet  combustion  would  simplify  it  sufficiently  to 
permit  its  more  general  adoption.  In  the  meantime,  it  stands  as 

BBer.  34,  2991-2   (1901)  ;  S5,  3256:  4429   (1902)  ;  S6,  1937   (1903). 
«J.  Ind.  Eng.  Chem.  5,  398   (1913)  ;  6,  459-62   (1914)  ;  9,  139-40   (1917). 
7  India  Rubber  World,  57,  17-8  (1917)  ;  Bureau  of  Standards  Technologic  Paper 
145   (1919), 


80  THE  ANALYSIS  OF  RUBBER 

our  only  accurate  direct  method  for  the  determination  of  rubber, 
irrespective  of  the  condition  of  the  rubber  product,  and  the  degree 
of  vulcanization.  The  method  is  as  follows: 

The  preliminary  extractions  with  acetone,  chloroform,  and 
alcoholic  potash  will  show  whether  mineral  rubbers  or  oil  sub- 
stitutes are  present.  If  the  former,  then  acetone  and  chloroform 
extractions  are  necessary;  and  with  oil  substitutes,  make  acetone 
and  alcoholic  potash  extractions;  and  if  both  are  present,  make 
all  three  extractions.  When  an  alcoholic  potash  extraction  is 
made,  wash  the  sample  thoroughly  with  5%  hydrochloric  acid, 
hot  water  and  alcohol. 

Take  0.500  gr.  of  the  finely  ground  sample  (call  this  weight 
W),  and  extract  with  acetone  for  eight  hours  (make  other  extrac- 
tions, if  necessary,  as  stated  above) .  Dry  the  residue  in  hydrogen 
(or  other  inert  gas)  for  two  hours  at  100C.  Place  the  sample  in 
50  to  75  cc.  of  chloroform  and  allow  it  to  swell.  Pass  into  this, 
until  the  green  color  which  is  formed  persists  for  30  minutes,  the 
gases  formed  by  heating  arsenic  trioxide  and  nitric  acid  of 
specific  gravity  1.30.  To  avoid  contamination,  it  is  important 
that  no  rubber  connections  be  used.  Immerse  the  flask  contain- 
ing the  rubber  in  cold  water  during  the  nitration.  Allow  the  solu- 
tion to  stand  overnight;  the  next  day.  filter  off  the  nitrosite 
through  a  Goooh  crucible  and  wash  with  small  quantities  of 
chloroform.  Remove  the  acid  gases  and  chloroform  from  the 
flask  by  means  of  a  gentle  current  of  air.  Evaporate  the  filtrate 
to  dryness.  Dissolve  the  nitrosite  remaining  in  the  flask  in  the 
Gooch  crucible  and  in  the  residue  from  the  filtrate  in  acetone, 
and  filter  the  solution  through  asbestos  into  a  weight-burette. 
The  total  volume  should  be  about  100  cc.  Allow  this  solution  to 
stand  for  a  short  time  to  permit  any  sediment  which  may  form  to 
settle  out  in  the  bottom  of  the  weight  burette.8  Weigh  the 
burette  before  and  after  filling,  calling  the  difference  N.  Draw 
off  about  25  cc.  into  a  small  Erlenmeyer  flask,  reweigh  the 
burette,  and  call  the  difference  0.  Evaporate  the  portion  drawn 
off  to  a  small  volume,  transfer  to  a  porcelain  boat  (about  14  cm. 
lon^  and  1  cm.  wide),  which  has  been  filled  with  alundum,  and 
wash  flu*  flask  with  acetone  (it  is  best  to  make  this  transfer  in 
small  portions,  drying  the  boat  and  contents  for  a  few  minutes  be- 

*  The  nsunl  type  of  weight  burette  will  not  answer;  it  is  necessary  to  have 
Hie  solution  drawn  off  at  the  side,  about  an  inch  from  the  bottom.  See  B.  of 
S.  Tech.  Paper  14f>  for  a  sketch  of  the  weight  burette. 


THE  DETERMINATION  OF  RUBBER  81 

tween  each  addition).  After  the  final  washing  and  drying,  add  1 
or  2  cc.  of  1  %  solution  of  ammonia  in  distilled  water,9  and  dry  in 
an  inert  gas  for  one  hour  at  90C.  Repeat  with  a  second  portion  of 
ammonia  and  dry  as  before.  By  this  means,  all  of  the  organic 
solvent  will  be  removed. 

Place  the  boat  in  the  furnace,  and  proceed  with  the  combus- 
tion. Pass  the  products  of  combustion  through  U-tubes  or  other 
satisfactory  absorption  tubes,  placed  in  the  following  order:  a, 
b,  c,  potassium  bichromate-cone,  sulfuric  acid;  d,  20-mesh 
powdered  zinc ;  e,  f ,  soda-lime  and  calcium  chloride ;  g,  potassium 
bichromate-cone,  sulfuric  acid;  h,  dilute  palladium  chloride  solu- 
tion, (very  little  palladium  chloride  is  needed;  use  about  a  drop 
of  a  10%  solution  in  10  cc.  of  water).  Weigh  e,  f,  and  g  before 
and  after  each  combustion;  refill  c  and  g  frequently  from  the 
same  stock  solution,  so  that  the  gases  which  enter  e,  and  those 
that  leave  g,  will  have  the  same  moisture  content.  The  pal- 
ladium chloride  serves  to  detect  the  presence  of  carbon  monoxide 
or  other  reducing  gases;  if  there  is  any  blackening,  it  shows  in- 
complete oxidation,  and,  in  this  event,  the  results  must  be  dis- 
carded and  the  determination  repeated. 

The  carbon  dioxide  will  equal  the  algebraic  sum  of  the  differ- 
ences in  tubes  e,  f,  and  g.  Call  this  sum  P.  The  factor  for  cal- 
culating from  carbon  dioxide  to  rubber  hydrocarbons,  is  .309. 
The  formula  is  therefore  as  follows: 

P  X  0.309  X  N  X  100 

n        w =  %  of  rubber  hydrocarbons. 

u  x  vv 

Correct  this  figure  for  whatever  extractions  were  made  previ- 
ous to  nitration.10 

The  Indirect  Methods. 

The  indirect  methods  comprise  those  in  which  the  rubber  is 
decomposed  and  rendered  soluble  in  some  solvent,  and  the  ma- 
terial which  goes  into  solution  is  called  rubber  hydrocarbons.  A 
great  many  solvents  have  been  suggested,  heavy  petroleum  (B.P. 
230-280) ,  anisole,  phenetole,  xylol,  paraffin  oil,  camphor  oil,  tere.- 

•  The  nitrosite  precipitate  is  insoluble  in  distilled  water,  and  in  acids ;  it  Is 
soluble  in  aqueous  alkaline  solutions.  Ammonium  hydroxide  is  used  simply  be- 
cause it  provides  the  necessary  alkalinity,  is  volatile,  and,  even  If  It  Is  not 
completely  driven  off,  introduces  no  error  whatsoever. 

10  In  case  the  extractions  contain  other  material  than  rubber,  the  correction 
applied  must  be  an  arbitrary  one.  It  will  be  recalled  that  the  true  resin  con- 
tent of  high  grade  rubber  is  about  4%  or  less,  and  the  normal  chloroform  and 
alcoholic  potash  extracts  of  rubber  are  about  1%  and  1%  respectively. 


82  THE  ANALYSIS  OF  RUBBER 

bene,  turpentine,  salol,  nitrobenzene,  aniline,  cumene  and  cymene. 
The  general  procedure  is  the  same  for  all  of  them;  the  acetone, 
chloroform  and  alcoholic  potash  extractions  remove  the  soluble 
organic  fillers;  the  solvent  is  then  added  and  heated  until  the 
rubber  passes  into  solution.  The  fillers  are  separated  by  filtration 
and  weighed,  and  the  loss  is  vulcanized  rubber.  Sometimes 
special  steps  are  taken  to  bring  the  fillers  into  a  state  in  which 
they  can  be  easily  filtered.  All  sorts  of  solvents  are  used  to 
wash  the  fillers. 

The  principal  objection  to  these  methods  is  that  one  can  never 
be  sure  that  there  are  no  organic  or  inorganic  fillers  passing  into 
solution,  or  that  insoluble  organic  compounds  are  not  formed 
from  the  rubber  and  solvent.  Correction  after  correction  must  be 
applied,  until  finally,  instead  of  a  determination  of  rubber,  we 
have  almost  a  complete  system  of  analysis. 

Difference  Methods. 

There  is  really  no  sharp  line  dividing  the  difference  methods 
from  the  indirect  methods,  but  we  have  reserved  the  term  "differ- 
ence" methods  for  those  which  remove  the  rubber  by  ignition. 
Such  methods  are  widely  used,  largely  because  they  are  rapid, 
and  it  is  a  fact  that  they  are  frequently  quite  accurate,  espe- 
cially in  the  absence  of  organic  fillers.  In  most  cases,  the  sample 
is  subjected  to  acetone,  chloroform  and  alcoholic  potash  extrac- 
tions, and  then  ignited.  At  times,  only  the  acetone  extraction  is 
made.  A  more  complicated,  and  more  exact  procedure,  is  that 
of  the  Joint  Rubber  Insulation  Committee.11  This  method  is 
intended  for  30%  and  40%  insulation  compounds,  but  may  be 
used  for  any  compound  containing  no  organic  fillers.  Mineral 
rubber,  lampblack,  glue,  cellulose,  etc.,  all  give  high  results.  The 
method  is  as  follows: 

Add  to  the  rubber  residue  from  the  alcoholic  potash  extrac- 
tion, sufficient  water  to  make  the  total  volume  125  cc.,  and  then 
add  25  cc.  cone,  hydrochloric  acid.  Heat  for  one  hour,  decant 
through  a  Buchener  funnel,  using  hardened  paper,  and  wash  with 
25  cc.  of  hot  water;  repeat  this  treat  twice.  The  rubber  should 
be  white,  and  free  from  black  specks  from  undissolved  fillers 
(lead  sulfide).  Wash  the  rubber  free  from  chlorides,  transfer  the 
rubber  to  the  filter  paper,  dry  as  much  as  possible  by  suction, 

"Am.  Inst.  Elec.  Eng.,  April,  1917. 


THE  DETERMINATION  OF  RUBBER  83 

wash  with  50  cc.  of  95%  alcohol,  and  transfer  the  entire  residue 
to  a  weighing  bottle.  Dry  to  constant  weight  at  95  to  100C.  Let 
this  weight  be  represented  by  C. 

On  a  portion  D  of  this  residue  C,  determine  the  ash  E  (see 
below)  and  the  sulfur  F  in  ash  E.  Determine  the  sulfur  H  in 
another  portion  G  of  residue  C. 

To  determine  E,  place  about  0.50  of  residue  C  into  a  weighed 
porcelain  crucible,  heat  gradually  until  the  crucible  has  ceased 
to  smoke,  then  raise  the  temperature  and  heat  until  all  organic 
matter  is  destroyed.  Cool  and  weigh,  calling  the  residue  E.  If  E 
is  small,  the  determination  of  sulfur  in  the  ash  may  be  omitted, 
and  F  assumed  to  be  zero.  From  the  data  thus  obtained,  the 
percentage  of  rubber  hydrocarbons  in  the  original  material  is 
calculated  as  follows: 

100  C    /-  E  — F     —  H    \        .,  .    ,,       ,     , 

— 1  1 —       — :_ -p-     }        7°  rubber  hydrocarbons. 

The  simplest  method  of  all,  is  the  determination  of  the  ash. 
'Wrap  a  1  gr.  sample  in  filter  paper,  and  extract  with  acetone  for 
four  hours;  ignite  the  residue  in  a  porcelain  crucible;  cool  and 
weigh.  Correct  for  the  sulfur  in  the  ash  by  adding  a  few  drops 
of  nitric  acid-bromine  mixture,  heat  on  the  steambath,  then  add 
5  gr.  sodium  carbonate,  dry  carefully  until  all  moisture  is 
removed,  fuse  the  residue,  extract  with  hot  water  and  filter;  make 
the  filtrate  just  acid  with  hydrochloric  acid,  heat  to  boiling,  add 
barium  chloride  solution,  and  determine  the  barium  sulfate  as 
usual.  Deduct  the  sulfur  thus  found  from  the  residue  on  ignition, 
and  the  difference  is  called  "ash,  sulfur  free." 

In  the  ash  method,  rubber  is  the  difference  between  100%  and 
the  sum  of  the  total  sulfur,  ash  (sulfur  free),  and  any  other 
determinations  of  fillers  which  may  have  been  made. 

The  only  good  word  which  can  be  said  for  the  ash  determina- 
tion as  a  correct  method  for  the  determination  of  rubber,  is  that 
it  requires  only  a  crucible,  a  Bunsen  burner,  and  a  balance.12 

12  It  is  quite  within  reason  that  those  analysts  who  have  for  years  been 
adhering  to  the  "ash  method,"  will  find  fault  with  this  statement.  With  a  few 
simple  compounds,  satisfactory  results  may  be  obtained,  but  beyond  this  nothing 
is  certain.  Our  experience  with  this  method,  in  testing  rubber  materials  for  the 
U.  S.  Government  over  a  number  of  years,  demonstrated  only  too  clearly  how 
easy  it  was  to  be  led  astray  by  results  obtained  with  the  ash  method.  The 
errors  in  this  ash  method  are  frequently  very  large — it  is  only  the  occasional 
determination  which  comes  with  2  or  3%  of  the  truth.  The  plain  truth  of  the 
matter  is  that  the  ash  method  is  used  because  it  involves  little  labor,  and  re- 
quires but  a  short  time  to  complete,  but  if  a  reasonable  degree  of  accuracy  is  not 
necessary,  one  is  tempted  to  ask  "Why  make  the  determination  at  all?" 


Chapter  VIII. 
Sulfur  Determinations. 
Total  Sulfur. 

Sulfur  may  occur  in  rubber  compounds  in  any  of  the  follow- 
ing forms: 

(a)  Free  sulfur. 

(6)   Sulfur  combined  with  the  rubber. 

(c)  Sulfur  in  organic  compounds   (mineral  rubber,  oil  sub- 
stitutes, accelerators) . 

(d)  Sulfides  (zinc,  antimony,  mercury,  lead,  cadmium). 

(e)  Sulfites  and  sulfates   (calcium,  barium,  lead). 

In  addition  to  the  above,  there  are  other  substances,  such  as 
barium  carbonate,  lead  oxide,  etc.,  which,  while  not  containing 
sulfur,  are  important  factors  in  deciding  what  method  is  satis- 
factory for  the  determination  of  sulfur.  We  may  say  that  all 
rubber  compounds  will  contain  classes  (a)  and  (6),  but  beyond 
that,  nothing  definite  may  be  assumed.  It  is  obvious  that  if  many 
of  these  sulfur-bearing  substances  are  present,  the  determination 
of  total  sulfur  becomes  a  difficult  proposition,  and,  moreover,  the 
results  obtained  decrease  in  value. 

In  looking  over  the  foregoing  list,  it  is  obvious  that  three  steps 
are  essential:  (1)  oxidation  of  organic  and  inorganic  substances; 
(2)  fusion  of  inorganic  substances;  (3)  separation  of  metals 
forming  insoluble  sulphates  by  filtration  of  the  alkaline  solution 
of  the  fusion. 

The  purpose  of  the  oxidation  is  obvious;  the  oxidizing  agent 
must  be  sufficiently  effective  to  oxidize  rather  large  amounts  of 
free  sulfur  (frequently  5%,  and  possibly  10%  or  12%).  Since  it 
is  assumed  that  oxidation  will  be  accompanied  or  followed  by 
fusion,  it  is  not  essential  that  the  oxidizing  treatment  be  carried 
to  the  point  of  the  complete  oxidation  of  all  of  the  organic  sub- 
stances. The  period  of  fusion  must  suffice  to  convert  insoluble 

84 


SULFUR  DETERMINATIONS  85 

into  soluble  sulfates.  These  steps  are  fairly  well  agreed  upon, 
although  various  means  are  suggested  for  this  part  of  the  proce- 
dure. The  third  step,  filtration,  is  still  a  subject  of  controversy, 
although  why  it  should  be  so  is  difficult  to  understand.  If  the 
solution  is  acidified  before  filtration,  we  may  expect  to  form 
calcium,  lead,  and  barium  sulfates.  Calcium  sulfate  is  highly 
soluble,  lead  appreciably  so,  and  barium  sulfate  scarcely  at  all. 
Calcium  may  have  been  present  originally  as  the  carbonate 
(whiting) ,  or  as  the  sulphate  (in  antimony  compounds) ;  the  lead 
as  oxide,  sulfide,  sulfite,  or  sulfate,  and  barium  as  the  carbonate 
or  sulfate.  The  purpose  of  the  filtration  from  an  acid  solution  is 
to  thus  eliminate  the  insoluble  sulfates  originally  present  in  the 
rubber  mixture,  but  this  will  be  accomplished  only  when  barium 
sulfate  is  the  only  sulfate  originally  present,  and  lead  and  calcium 
are  present  in  very  small  amounts,  and  barium  carbonate  is 
absent.  It  is  evident  that  only  on  rare  occasions  will  the  condi- 
tions be  such  as  to  permit  the  filtration  of  the  solution  of  the 
melt  from  an  acid  solution,  and  with  compounds  of  unknown 
composition,  it  is  impossible. 

In  the  earliest  attempts  to  determine  the  sulfur  in  rubber, 
Henriques  oxidized  with  cone,  nitric  acid.  Later,  Alexander 
suggested  sodium  peroxide;  Hinrichsen,  a  modification  of  Gas- 
parini's  electrolytic  oxidation  (afterwards  improved  by  Spence) ; 
Waters  and  Tuttle  employed  cone,  nitric  acid  and  bromine; 
Pontio,  manganese  peroxide ;  Frank  and  Markwald,  fuming  nitric 
acid;  and  Kaye  and  Sharpe  fused  directly  with  zinc  oxide  and 
potassium  nitrate.  Some  advocate  a  solution,  others  direct  fusion, 
while  practically  all  of  those  who  had  a  preliminary  wet  oxida- 
tion added  a  fusion  later.  Solution  without  fusion  is  obviously  a 
faulty  procedure  in  the  presence  of  lead  and  barium  salts,  and  no 
data  have  as  yet  been  presented  to  show  that  the  direct  fusion 
methods  are  accurate  when  the  free  sulfur  is  high.  Of  the 
methods  employing  both  solution  and  fusion,  that  of  Waters  and 
Tuttle  has  given  the  most  consistent  results  with  all  types  of 
compounds,  especially  when  some  minor  changes  from  that 
originally  proposed  are  employed.  The  method  recommended  is 
as  t  follows: 

Place  0.500  gr.  of  rubber  in  a  porcelain  crucible  of  about  50  cc. 
capacity,  add  20  cc.  of  cone,  nitric  acid  saturated  with  bromine, 
cover  the  crucible  with  a  watch  glass,  and  allow  it  to  stand  for 


86  THE  ANALYSIS  OF  RUBBER 

one  hour.  Heat  the  crucible  gently  for  one  hour,  then  remove 
the  watch  glass,  rinsing  it  with  little  water,  and  evaporate  the 
solution  to  dryness  (with  pure  gum  compounds  before  evaporat- 
ing add  0.1  to  0.2  gr.  of  potassium  nitrate).  Add  5  gr.  of  1-1 
mixture  of  sodium  carbonate  and  potassium  nitrate,  and  1  or  2 
cc.  of  distilled  water;  digest  for  a  few  minutes,  and  then  spread 
the  paste  along  the  sides  of  the  crucible,  and  dry  on  a  steambath. 
Fuse  the  mixture,  being  careful  to  avoid  contamination  of  sulfur 
from  the  flame.  When  the  fusion  is  cold,  place  the  crucible  and 
contents  in  a  beaker  with  about  250  cc.  of  water  and  heat  for 
several  hours ;  filter  off  the  insoluble  carbonates,  washing  with  hot 
water.  The  total  volume  of  the  filtrate  should  be  between  300 
and  40  cc.  Add  7  to  8  cc.  cone,  hydrochloric  acid,  cover  the 
beaker,  and  heat  on  the  steambath.  Add  10  cc.  10%  barium 
chloride,  and  allow  to  stand  overnight;  filter  off  the  precipitated 
barium  sulfate,  ignite  carefully  over  a  Bunsen  flame,  cool  and 
weigh.  Calculate  to  sulfur,  using  the  factor  0.1373. 

The  principal  change  from  the  published  method  is  the  addi- 
tion of  the  potassium  nitrate  before  evaporating  off  the  nitric 
acid;  it  is  necessary  only  in  the  absence  of  any  basic  fillers,  and 
serves  the  purpose  of  changing  any  free  sulfuric  acid  into  potas- 
sium sulfate.  With  sulfuric  acid,  there  is  some  danger  of  it  being 
reduced  by  the  organic  matter,  and  sulfur  lost  as  S02. 

Probably  the  weakest  feature  of  this  method  is  the  wear  and 
tear  on  the  crucibles,  if  this  indeed  can  be  considered  a  weak 
point.  There  are  some  makes  of  crucibles  which  will  not  last 
through  a  determination;  on  the  other  hand,  some  American  and 
Japanese  crucibles  last  through  five  to  ten  fusions.  At  this  rate, 
the  cost  is  negligible.  It  might  be  added  further,  in  speaking  of 
crucibles,  that  the  smaller  and  thinner  crucibles  last  longer  than 
the  thicker  ones;  much  of  the  crucible  trouble  has  been  due  to  the 
use  of  extra  large  and  thick  crucibles,  and  to  the  use  of  inferior 
makes. 

Occasionally,  one  finds  the  statement  that  the  solution  of  the 
fusion  should  be  evaporated  to  dryness  with  hydrochloric  acid 
to  get  rid  of  the  last  traces  of  nitric  acid,  and  to  render  insoluble 
the  silica  in  the  alkali  silicates  which  have  been  formed.  The 
small  amounts  of  nitrates  remaining  after  the  fusion  will  not 
appreciably  affect  the  accuracy  of  the  determination  and 


SULFUR  DETERMINATIONS  87 

dehydration  of  the  silica  is  unnecessary,  as  Hillebrand 1  has 
shown.  However,  it  is  essential  that  the  alkaline  solution  should 
be  reasonably  cool  when  the  acid  is  added,  that  undue  excess  of 
acid  be  avoided,  and  that  the  solution  be  not  allowed  to  concen- 
trate to  any  large  extent.  We  have  frequently  found  consider- 
able amounts  of  silica  when  the  solution  containing  the  precipi- 
tated barium  sulfate  had  been  allowed  to  concentrate  to  50  cc. 
or  less. 

Allen  and  Johnston  2  have  shown  that  the  precipitate  of  barium 
sulfate  formed  in  the  presence  of  alkali  chlorides  and  hydrochloric 
acid,  is  contaminated  with  chlorine  and  alkalies,  and  have 
worked  out  a  method  for  correcting  these  errors,  and  so  arrive 
at  the  true  sulfur  value.  The  precipitate  obtained  in  the  deter- 
mination of  total  sulfur  in  rubber  is  subject  to  these  same  errors, 
but  if  the  barium  chloride  be  added  rapidly  to  the  hot  solution, 
the  solution  never  heated  to  boiling,  and,  further,  if  it  is  allowed  to 
stand  for  at  least  18  hours  before  filtering,  the  contamina- 
tion will  be  low,  and  the  fortunate  balancing  of  errors  will  give 
results  very  close  to  the  truth,  so  much  so  that  it  will  not  ordina- 
rily pay  to  take  the  time  for  the  corrections  suggested  by  Allen 
and  Johnston.  It  should  be  noted,  however,  that  any  attempt  to 
improve  the  method  of  precipitation  by  eliminating  only  one  of 
the  errors,  will  yield  results  which  are  not  as  accurate  as  if  the 
method  was  not  changed. 

If  the  free  sulfur  is  low,  the  fusion  method  of  the  Joint  Rubber 
Insulation  Committee 3  will  be  found  acceptable: 

Mix  0.500  gr.  of  rubber  with  4  gr.  sodium  peroxide  and  6  gr. 
potassium  carbonate  in  a  dry  15  cc.  iron  crucible,  and  cover. 
Insert  the  crucible  in  a  hole  in  a  heavy  brass  plate  so  that  about 
two  thirds  of  the  crucible  projects  through  the  hole.  Heat  cau- 
tiously until  the  first  part  of  the  reaction  has  taken  place,  and 
then  increase  the  heat  until  the  mixture  fuses.  Remove  the 
flame  and  cool;  place  the  crucible  and  cover  in  a  porcelain  cas- 
serole containing  200  cc.  of  water,  add  5  to  10  cc.  of  bromine 
water,  and  boil  until  the  melt  is  dissolved.  Allow  the  precipitate 

1  Analysis  of  silicate  and  carbonate  rocks,  U.  S.  Geological  Survey  Bull.  422, 
p.  198. 

2  J.  Am.  Chem.  Soc.  82,  588-617   (1910)  ;  see  also  Richards  and  Parker,  Proc. 
Am.  Acad.  SI,  67   (1896)  ;  Hulett  and  Duschak,  Z.  Anorg.  Chem.  40,  196   (1904)  ; 
John  Johnston  and  L.  H.  Adams,  J.  Am.  Chem.  Soc.  33,  829-45   (1911). 

3J.  Ind.  Eng.  Chem.  6,  75-82   (1914). 


88  THE  ANALYSIS  OF  RUBBER 

to  settle,  decant  the  solution  through  a  thick  filter  and  wash  with 
hot  water.*  Make  the  filtrate  faintly  acid  with  hydrochloride 
acid,  heat  to  boiling,  add  10  cc.  of  10%  barium  chloride  solution, 
allow  to  stand  overnight;  filter  the  barium  sulfate  as  usual. 

This  method  was  originally  recommended  for  testing  insulated 
wire,  in  which  the  free  sulfur  was  limited  to  0.7  %,  and  was  found 
quite  satisfactory.  Tuttle  and  Isaacs 5  found  that  with  high  free 
sulfur,  the  results  obtained  were  not  accurate.  It  has  been  sug- 
gested that  by  increasing  the  quantities  of  sodium  peroxide  and 
potassium  carbonate,  even  these  high  free  sulfur  samples  could 
be  analyzed  without  any  trouble,  but  data  are  lacking  in  support 
of  this  contention. 

It  would  be  extremely  desirable,  from  the  time  and  labor-sav- 
ing points  of  view,  if  the  oxidation  of  the  free  sulfur  and  the 
fusion  could  be  accomplished  in  one  treatment.  Spence's 6  elec- 
trolytic method  eliminates  the  fusion,  in  the  absence  of  lead  and 
barium  salts.  Evans  and  Merling 7  have  devised  a  method, 
using  a  Parr  calorimeter:  0.200  gr.  of  rubber  is  packed  in  sodium 
peroxide,  with  some  sugar  and  potassium  chlorate.  The  ignited 
mass  is  extracted  with  water,  filtered,  acidified,  and  the  sulfur 
precipitated  as  usual.  The  authors  claim  to  have  secured  some 
excellent  results  so  far,  and  the  time  required  is  very  little,  but 
it  seems  desirable  that  others  test  this  method  to  discover  its 
limitations,  and  faults,  if  it  has  any. 

Free  Sulfur. 

The  procedure  to  be  adopted  for  determining  the  sulfur  in  the 
acetone  extract  depends  largely  upon  the  nature  of  the  material, 
and  whether  it  is  desired  to  make  further  separation  of  the 
constituents  in  the  extract.  If  not,  the  simplest,  and  yet  the  most 
accurate  method  we  have,  is  as  follows: 

To  the  dried  extract,  add  100  cc.  of  water,  and  3  to  5  cc.  of 
bromine.  (If  a  very  high  free  sulfur  is  indicated  by  the  character 
of  the  extract,  the  amount  of  bromine  should  be  increased.) 
Allow  the  flask  to  stand  for  half  an  hour  to  an  hour,  boil  off 
the  bromine,  and  when  the  solution  is  practically  colorless,  filter 

« The  original  method  called  for  dehydration  of  silica,  but,  as  previously  noted, 
this  is  unnecessary. 

•J.  Ind.  Eng.  Chem.  7,  658  (1913). 
•  J.  Ind.  Eng.  Chem.  k,  413  (1912). 
» India  Rubber  World,  64,  658  (1921). 


SULFUR  DETERMINATIONS  89 

through  a  folded  filter  into  a  small  beaker;  cover  the  beaker, 
heat  to  boiling,  add  10  cc.  of  10%  barium  chloride,  and  after 
standing  overnight,  determine  the  barium  sulfate  as  usual. 

This  method  determines  all  of  the  sulfur  in  the  extract ;  a  great 
many  checks  have  been  run  by  taking  the  insoluble  residue,  fus- 
ing it  with  sodium  carbonate  and  potassium  nitrate,  and  deter- 
mining the  sulfur  as  is  done  in  the  Waters  and  Tuttle  method,  but 
the  sulfur  has  never  exceeded  0.01  to  0.02%  in  this  residue.  The 
oxidation  is  complete,  rapid,  requires  no  evaporation,  furnishes 
its  own  acidity  by  the  reaction  with  the  barium  chloride ;  in  fact, 
after  ten  years,  there  still  remains  to  be  found  some  objection  to 
its  use. 

If  it  is  desired  to  make  further  examination  of  the  acetone 
extract,  the  method  of  the  Joint  Rubber  Insulation  Committee 8 
is  recommended:  the  method  starts  where  the  acetone  extraction 
has  been  treated  with  alcoholic  potash,  the  alcohol  removed,  the 
residue  taken  up  in  water,  extracted  with  ether,  and  the  ether 
washed  with  water. 

To  the  aqueous  solution,  add  2  gr.  potassium  nitrate;  evaporate 
to  dryness  in  a  nickel  or  silver  dish,  and  heat  to  quiet  fusion. 
Transfer  to  a  beaker,  neutralize  with  hydrochloric  acid,  add  2  cc. 
of  acid  in  excess,  filter  and  wash,  making  the  filtrate  up  to  200  cc. 
Heat  to  boiling,  add  a  slight  excess  of  barium  chloride  solution, 
allow  to  stand  overnight,  and  determine  the  barium  sulfate  as 
usual. 

Kelly  9  calls  attention  to  the  fact  that  what  we  have  been  deter- 
mining as  free  sulfur  is  not  the  true  free  sulfur,  but  includes,  in 
addition  to  the  sulfur  which  may  be  said  to  be  available  for 
further  vulcanization,  such  amounts  of  sulfur  which  may  have 
been  combined  with  the  organic  resins  extracted  by  acetone. 
Obviously,  this  is  so,  and  in  the  data  presented  by  Kelly,  which, 
however,  covers  only  one  compound,  there  is  about  0.40%  of 
sulfur  combined  with  the  organic  matter  in  the  extract.  In  such 
cases,  the  free  sulfur  as  determined  in  the  past  is  quite  misleading. 
It  is  still  a  very  great  question  as  to  whether  the  sulfur  will 
always  be  of  the  same  order  of  magnitude  as  Kelly  indicates.  It 
would  have  been  very  helpful  if  commercial  samples  had  been 

•Loc.  cit. 

•  The  determination  of  the  true  free  sulfur,  and  the  true  sulfur  of  vulcaniza- 
tion;  J.  Ind.  Eng.  Chem.  12,  875-8  (1920). 


90  THE  ANALYSIS  OF  RUBBER 

treated — in  the  many  samples  we  have  tested,  we  have  frequently 
found  samples  where  the  free  sulfur,  as  determined  by  the  bro- 
mine method,  was  less  than  0.10%.  Upton10  gives  figures  for 
free  sulfur  on  some  samples  of  insulated  wire,  determined  by  two 
methods,  and  several  of  these  were  below  0.20%,  and  in  one  case 
only  0.07%.  Without  questioning  the  force  of  Kelly's  argument, 
it  does  seem  as  though  we  needed  more  data  to  show  what  varia- 
tions there  are  between  the  free  sulfur  as  at  present  determined, 
and  the  amount  he  calls  the  true  free  sulfur. 

Kelly's  method  is  as  follows: 

The  acetone  extraction  is  performed  as  usual.  The  acetone  is 
driven  off  at  not  over  65C.  To  the  residue,  add  50  cc.  of  75% 
alcohol  which  has  been  saturated  with  sulfur.  Weigh  the  flask 
and  contents  to  0.5  gr.  Heat  for  a  few  minutes,  to  get  the  organic 
matter  in  solution,  and  then  cool  slowly.  Allow  to  stand  three 
hours;  reweigh,  and  add  75%  alcohol,  sulfur-free,  to  replace  the 
loss.  Decant  the  solution,  wash  two  or  three  times  with  75% 
alcohol  saturated  with  sulfur,  and  then  dry.  The  sulfur  may 
then  be  determined  by  any  satisfactory  method. 

No  word  is  said  as  to  what  is  to  be  done  with  the  alcoholic 
solution  of  the  resins;  no  scheme  has  been  given  for  weighing 
them,  and  at  65C  the  extract  would  not  be  sufficiently  dried  to 
take  that  figure  as  the  total  acetone  extract.  If  one  knew  just 
how  much  alcohol  was  used,  and  the  sulfur  it  contained,  the 
solution  could  be  evaporated  to  dryness  in  a  weighed  flask,  dried 
to  constant  weight  at  90C,  and  the  organic  extract  determined 
after  making  due  allowance  for  the  sulfur  in  the  alcohol.  As  it 
stands  now,  the  new  method  requires  a  separate  extraction  for 
the  acetone  extract. 

Caspari lx  gives  a  similar  method  to  the  one  used  by  Kelly. 
He  uses  petroleum  spirit,  boiling  point  60-100C,  which  dissolves 
3.0  gr.  of  sulfur  per  litre,  whereas  the  75%  alcohol  dissolves  only 
0.08  gr.  Kelly  says  nothing  about  the  accuracy  when  mineral 
rubber,  tars,  or  paraffins  are  present,  whereas  Caspari  speaks  of 
the  difficulty  in  getting  these  in  solution.  However,  as  we  are 
interested  only  in  separating  the  true  free  sulfur  from  the  sulfur- 
carrying  organic  substances,  it  is  quite  within  reason  that  undis- 
solved  paraffin  would  create  no  impassable  difficulties. 

">J.  Ind.  Eng.  Chem.  10,  518   (1918). 

11  India  Rubber  Laboratory  practice,  p.  116. 


SULFUR  DETERMINATIONS  91 

Sulfur  of  Vulcanization. 

It  is  often  desirable  to  know  the  amount  of  sulfur  actually 
combined  with  the  rubber  during  the  process  of  vulcanization, 
both  as  regards  determining  the  extent  to  which  it  has  proceeded 
and  to  attain  a  greater  uniformity  in  manufacturing  practice. 
The  simplest  method  for  estimating  uniformity,  for  comparative 
results,  is  by  means  of  stress-strain  curves,  but  mechanical  de- 
fects operate  to  change  values,  so  that  comparisons  are  at  best 
difficult  and  uncertain.  The  noticeable  effect  on  the  vulcanization 
by  slight  changes  in  sulfur  content,  demonstrate  that  the  amount 
of  sulfur  which  actually  unites  with  the  rubber  is  the  controlling 
feature  of  the  vulcanization.  The  value  for  the  sulfur  of  vulcani- 
zation is  necessary  for  the  calculation  of  the  total  rubber  hydro- 
carbons in  some  of  the  direct  methods,  and  a  further  use  is  the 
possible  discovery  of  the  presence  of  reclaimed  rubber  in  a  rubber 
compound.12 

Several  possibilities  are  available,  depending  upon  the  nature 
of  the  rubber  compound.  The  simplest  case  is  that  of  pure  rub- 
ber and  sulfur,  and  occurs  but  seldom  in  commercial  articles, 
although  it  is  overworked  as  a  formula  for  determining  the  value 
or  properties  of  crude  rubber.  In  this  case,  if  the  total  sulfur  is 
S,  the  free  sulfur  SP,  the  percentage  of  rubber  100  —  S  then  the 
sulfur  coefficient,  Sv  will  be: — 

S  — SP 


In  samples  containing  no  organic  sulfur  compounds,  the  fol- 
lowing method,  based  upon  the  determination  of  sublimed  white 
lead  by  Schaeffer,13  gives  excellent  results: 

The  sample  is  extracted  with  acetone  for  eight  hours,  and  the 
free  sulfur  determined  in  the  extract  by  the  bromine  method. 
The  residue  is  placed  in  a  porcelain  boat,  and  transferred  to  a 

32  This  is  not  as  simple  a  proposition  as  it  was  before  the  rapid  accelerators 
came  into  use.  With  inorganic  accelerators,  the  proper  cure  for  rubber  was 
approximately  at  a  coefficient  of  3.0  to  3.5.  and  hence  higher  coefficients  were 
fair  indications  of  the  presence  of  reclaimed  rubber,  especially  in  connection 
with  other  qualitative  tests.  Today,  the  value  of  the  coefficient  of  vulcanization 
is  almost  nil,  when,  by  the  use  of  appropriate  accelerators,  good  cures  can  be 
obtained  with  sulfur  coefficients  below  2.0.  Of  course,  if  one  can  learn  what 
accelerator  has  been  used,  and  determine  the  coefficient  for  the  best  cures  with 
that  accelerator,  such  data  might  be  quite  valuable  in  determining  the  condition 
of  the  rubber  in  the  sample  under  observation. 

13  J.  Ind.  Eng.  Chem.  4,  837  (1912), 


92  THE  ANALYSIS  OF  RUBBER 

hard  glass  tube.  Carbon  dioxide  is  passed  through  the  tube, 
which  is  then  heated,  gradually  at  first,  and  then  at  a  dull  red 
heat  for  a  few  minutes.  The  organic  matter,  together  with  the 
rubber,  is  distilled  out,  but  the  mineral  sulfides  and  sulfates  are 
unchanged.  The  sulfur  in  the  fillers  is  determined -by  transferring 
the  residue  to  a  porcelain  crucible,  and  determining  the  sulfur 
therein  by  the  Waters  and  Tuttle  method  for  total  sulfur.  The 
calculations  for  this  method  require  a  separate  determination  of 
rubber,  R.  Calling  the  sulfur  in  the  residue  SR,  then  the  sulfur 
coefficient  will  be  calculated  as  follows: 

_S— (SF  +  SR) 

R 
This  is  the  same  formula  as  before,  when  R  =  100  —  S,  and 

SR:=0. 

The  most  difficult  case  is  when,  in  addition  to  sulfides  and  sul- 
fates, we  have  organic  substances  containing  sulfur,  such  as 
oil  substitutes,  mineral  rubber,  etc.  There  are  several  procedures 
which  may  be  followed,  but  the  safest  is  probably  to  use  Wes- 
son's nitrosite  method  as  revised  by  Tuttle  and  Yurow.14  In  his 
original  article,  Wesson  says:  "If  the  statement  of  Alexander15 
proves  to  be  true  that  the  sulfur  of  vulcanization  of  the  rubber 
remains  quantitatively  in  the  nitrosite,  this  method  could  pos- 
sibly admit  of  the  simultaneous  determination  of  the  sulfur  of 
vulcanization.  An  aliquot  portion  of  the  clear  acetone  solution 
of  the  nitrosite  would  be  evaporated  to  dryness,  and  the  sulfur 
determined  in  the  usual  way."  A  few  attempts  were  made  to 
determine  the  sulfur  of  vulcanization  in  this  way,  but  not  until 
after  the  errors  which  were  contained  in  Wesson's  method  had 
been  eliminated,  was  it  possible  to  secure  accurate  determination 
of  the  rubber,  and  until  then,  little  effort  was  made  to  determine 
the  sulfur  of  vulcanization.  When  the  final  revision  was  in  shape, 
determinations  of  the  sulfur  of  vulcanization  were  found  to  check 
very  well. 

The  sulfur  coefficient  figured  by  this  method,  is  the  result  ob- 
tained by  dividing  the  combined  sulfur  by  the  percentage  of 
rubber  hydrocarbons;  such  a  calculation  leaves  no  opening  for 

14  As  a  matter  of  fact,  this  method  can  be  used  for  any  compound,  and  Is  not 
confined  in  its  application  to  this  single  case  where  organic  sulfur  compounds 
are  present;  it  is  equally  effective  in  rubber  sulfur  mixtures,  and  with  mixtures 
containing  mineral  sulfur  bearing  fillers. 

10  Z.  Angew.  Chem,  20,  1364  (1907)  ;  2J,  687  (1911)  ;  Ber.  40,  1077   (1907), 


SULFUR  DETERMINATIONS  93 

questions  as  to  whether  or  not  the  sulfur  was  combined  with 
the  rubber,  or  with  something  other  than  rubber.  It  is  simple, 
direct,  and  accurate. 

When  possible  to  make  it,  the  direct  determination  of  the 
sulfur  coefficient  (or  for  that  matter  any  determination)  is  pref- 
erable to  the  difference  methods,  since  all  questions  regarding 
interfering  substances  are  eliminated.  Kelly  points  out  that 
not  only  is  the  figure  usually  determined  as  free  sulfur  really  a 
mixture  of  elemental  sulfur  and  sulfur  combined  with  the  resins 
and  other  soluble  constituents  of  the  rubber,  but  that  part  of 
the  sulfur  insoluble  in  acetone  is  soluble  in  alcoholic  potash. 
There  seems  to  be  no  doubt  that  our  use  of  the  term  free  sulfur 
is  not  exactly  correct;  and  that  some  of  the  residual  sulfur 
should  be  removed  by  alcoholic  potash  seems  equally  reasonable, 
but,  for  ordinary  length  cures,  the  amount  so  removed  is  small 
(Kelly  shows  0.07%  for  2%  hours) . 

If  we  figure  our  coefficient  on  only  the  sulfur  that  is  insoluble 
in  alcoholic  potash,  obviously  we  should  also  take  into  our  cal- 
culations the  non-rubber  constituents,  and  this  would  include 
the  acetone  soluble  matter,  or  resins.  In  our  formula,  we  would 
therefore  have  to  correct  R  for  the  acetone  extract  A,  and  the 
alcoholic  potash  extract  P,  and  we  would  have  to  deduct  the 
sulfur  in  the  alcoholic  potash  extract,  SP;  hence,  we  would  have 
the  rather  involved  equation: 

S— (SF  +  SR  +  SP) 
R-(A  +  P) 

As  a  matter  of  fact,  the  relative  amounts  of  rubber  and  non- 
rubber  substances  insoluble  in  acetone  are  such  that  even  making 
this  additional  correction  changes  the  coefficient  very  slightly, 
certainly  within  the  limits  of  experimental  error,  as  far  as  our 
experience  goes.  Hence,  although  no  doubt  the  published  data 
for  coefficients  of  vulcanization  are  not  absolute  values,  they  are 
probably  relatively  accurate,  and  are  comparable.  Hence  any 
deductions  which  may  have  been  made  from  these  data  are  no 
doubt  just  as  valid  as  though  every  correction  had  been  made. 

Sulfur  in  Fillers. 

The  sulfur  in  fillers  is  determined  as  given  under  the  method 
for  the  determination  of  rubber  by  the  ash  method  (cf.  page  83). 


Chapter  IX. 
Detection  of  Organic  Accelerators. 

There  is  very  little  published  work  on  this  subject;  probably 
a  few  laboratories  have  some  special  tests  of  their  own,  but  as 
yet  no  one  has  seriously  taken  up  this  field.  The  data  given 
below  is  largely  from  Twiss  and  Martin,1  and  Earle  L.  Reed.2 

Paranitrosodimethylaniline.  Extract  about  10  gr.  of  the 
sample  with  acetone,  and  dry  the  extract;  add  5  cc.  dilute  hydro- 
chloric acid,  shake  thoroughly,  and  filter.  A  pink  or  carmine 
color  results  if  p-nitrosodimethylaniline  is  present.  If  the  filtrate 
is  colorless,  divide  it  into  two  portions,  using  one  to  test  for 
aniline,  and  the  other  for  hexamethylenetetramine. 

The  above  test  is  a  better  negative  than  a  positive  test — if  no 
color  develops,  the  accelerator  is  not  present,  but  there  may  be 
other  organic  bases  which  will  give  a  pink  color  on  acidification 
with  dilute  hydrochloric  acid. 

Twiss  and  Martin  call  attention  to  the  color  of  the  acetone 
extract  which,  however,  is  too  common  a  color  to  use  as  an  indica- 
tion of  an  organic  accelerator.  A  more  positive  test  is  to  treat 
the  dried  acetone  extract,  or  a  dilute  hydrochloric  acid  extract 
of  a  finely  ground  sample,  with  hydrogen  sulfide  water  and  ferric 
chloride  solution,  forming  a  blue,  or  greenish-blue,  if  paranitro- 
sodimethylaniline  is  present.  The  reaction  depends  upon  the  re- 
duction of  part  of  the  accelerator  during  vulcanization,  to 
p-aminodimethylaniline,  which,  when  treated  as  stated,  forms 
methyleneblue. 

Twiss  gives  the  following  alternative  method:  treat  the  hydro- 
chloric acid  solution  of  the  dried  acetone  extract  with  a  small 
piece  of  metallic  zinc.  Filter  off  the  solution,  cool  thoroughly, 
and  add  a  well  cooled  dilute  aqueous  solution  of  sodium  nitrate. 
Add  a  small  amount  of  this  mixture  to  a  solution  of  beta-napthol, 
with  excess  of  aqueous  sodium  hydroxide.  A  deep  blue  results 
in  the  presence  of  p-nitrosodimethylaniline. 

'Rubber  Age,  9,  379-80   (1921). 
*  Unpublished  data. 

94 


DETECTION  OF  ORGANIC  ACCELERATORS       95 

It  can  also  be  tested  for  by  means  of  the  Liebermann  reaction. 
The  dried  acetone  extract  is  boiled  with  a  small  amount  of  dilute 
caustic,  and  filtered;  the  filtrate  is  evaporated  to  dryness,  cone. 
sulfuric  acid  and  phenol  added,  the  mixture  diluted  with  water, 
and  made  alkaline  with  caustic  potash,  when  a  deep  blue  colora- 
tion will  appear. 

Aniline.  Using  the  hydrochloric  acid  filtrate  after  testing  for 
paranitrosodimethylaniline,  add  a  drop  of  freshly  prepared  and 
filtered  solution  of  bleaching  powder.  A  violet  color  indicates 
the  presence  of  aniline.  Thiocarbanilide  will  ordinarily  give  no 
reaction  to  this  test,  unless  present  in  very  large  amounts.  It  is 
well,  in  order  to  make  sure  of  its  absence,  to  take  another  portion 
of  the  dried  extract,  and  heat,  and  look  for  the  characteristic 
odor  of  thiocarbanilide. 

Thiocarbanilide.  A  portion  of  the  dried  acetone  extract  is 
placed  in  a  test  tube,  stoppered,  and  connected  by  a  delivery 
tube  with  a  second  test  tube  containing  two  or  three  cc.  of  distilled 
water.  The  delivery  tube  must  dip  below  the  surface  of  the 
water.  The  first  test  tube  is  now  heated  until  bubbles  escape 
through  the  water  in  the  second  test  tube,  after  which  the  heat- 
ing is  continued  strongly  for  two  or  three  minutes.  Test  the 
water  in  the  second  test  tube  for  aniline  with  the  filtered  bleach- 
ing powder  solution;  a  violet  color  will  indicate  thiocarbanilide 
if  the  original  aniline  test  was  negative. 

Thiocarbanilide  has  a  very  characteristic  odor,  which  is 
especially  noticeable  when  heated.  Heat  the  dried  acetone  ex- 
tract, and  compare  the  odor  with  that  of  some  heated  thio  in  a 
second  test  tube. 

H .xamethylenetetramine.  Using  the  second  portion  of  the 
hydrochloric  filtrate  from  the  test  for  p-nitrosodimethylaniline, 
add  5  cc.  of  water,  1  cc.  of  phosphoric  acid,  a  small  amount  of 
phenylhydrazine  hydrochloride,  2  drops  of  10%  ferric  chloride 
solution,  and  2  drops  of  cone,  hydrochloric  acid.  A  cherry  red 
color  is  produced  by  the  formaldehyde  from  the  hexamethylene- 
tetramine. 

Extract  a  ground  sample  with  water,  and  test  the  extract  for 
ammonia  with  Nessler's  solution.  A  positive  test  indicates  alde- 
hyde ammonia  or  hexa — although  some  of  the  less  commonly 
used  accelerators  may  yield  small  amounts  of  ammonia,  and 
hence  respond  to  this  test. 


96  THE  ANALYSIS  OF  RUBBER 

Diphenylamine.  To  the  dried  acetone  extract  from  about  10 
gr.  of  finely  ground  sample,  add  2  cc.  of  cone,  sulfuric  acid,  and 
agitate  gently.  Add  a  small  crystal  of  sodium  nitrate — a  blue 
coloration  results  if  diphenylamine  is  present. 

This  test  can  be  made  directly  on  light-colored  compounds  by 
placing  a  few  drops  of  cone,  sulfuric  acid  on  the  rubber  to  be 
tested,  dipping  a  glass  rod  in  dilute  nitric  acid,  and  touching  it 
to  the  edge  of  the  sulfuric  acid. 

Quinoidine.  Treat  the  dried  acetone  extract  with  dilute  sul- 
furic acid;  quinoidine  gives  a  blue  fluorescence.  Rochelle  salts 
precipitate  the  tartrates  of  quinine  or  cinchonidine,  but  not 
quinidine.  A  saturated  solution  of  potassium  iodide  added  to  an 
acid  solution  gives  quinidine  hydroiodide.  Quinine  and  quinidine 
give  the  thalleioquin  test,  but  cinchonine  and  cinchonidine  do 
not;  to  a  solution  of  the  acetone  extract  in  dilute  sulfuric  acid, 
add  very  weak  bromine  water,  drop  by  drop,  until  a  faint  yellow 
persists,  but  avoid  an  excess  of  bromine;  add  ammonia,  drop  by 
drop,  when  a  brilliant  green  color  results.  Making  this  solution 
acid  turns  the  color  to  red. 

General  Tests.  Extract  10  gr.  of  finely  ground  sample  with 
dilute  hydrochloric  acid,  cool  thoroughly,  and  diazotize  with  cold 
dilute  aqueous  sodium  nitrite.  (The  simplest  scheme  is  to  put  a 
small  piece  of  ice  in  the  solution  during  the  diazotizing;  it  can 
be  removed  later.)  After  a  few  minutes,  pour  a  little  of  this 
mixture  into  a  solution  of  beta-napthol  in  excess  of  caustic  soda ; 
a  red  precipitate  or  coloration  indicates  the  presence  of  a  primary 
aromatic  amine,  such  as  aniline  toluidine,  p-phenylenediamine, 
etc.,  or  of  derivatives  of  such  bases  with  aldehydes  (formaniline, 
methyleneaniline,  benzylidene-aniline) ,  and  with  carbon  bisulfide 
(thiocarbanilide,  or  triphenylguanidine) . 


Chapter  X. 
Mineral  Analysis. 

The  first  step  in  a  fillers  determination  of  a  rubber  compound 
is  to  make  a  qualitative  analysis  of  the  metals  which  it  contains. 
In  this  work,  the  color  of  the  sample  will  be  of  considerable 
assistance  in  cutting  out  unnecessary  steps,  as  will  also  a  knowl- 
edge of  the  use  to  which  the  article  is  to  be  put.  Only  in  the 
black  compounds  is  there  any  necessity  for  making  a  fairly  com- 
prehensive examination. 

Preparation  of  the  Solution.  The  possible  presence  of  lead, 
barium  and  calcium  in  a  mixture  containing  sulfur  (as  sulfuric 
acid)  makes  the  problem  of  making  up  a  solution  for  qualitative 
analysis  quite  an  interesting  one.  While  several  choices  are  open, 
the  following  procedure  is  recommended  because  of  the  fact  that 
it  permits  quantitative  separations  to  be  made  on  a  number 
of  elements: 

Place  exactly  2.500  gr.  of  finely  divided  rubber  in  a  porcelain 
casserole  (about  250  cc.  capacity),  cover  with  25  cc.  of  fuming 
nitric  acid,  and  after  standing  in  the  cold  for  15  to  30  minutes, 
covered  with  a  watch  glass,  heat  on  a  steam  bath  or  hot  plate 
until  the  rubber  and  all  other  organic  matter  is  entirely  de- 
stroyed. Potassium  chlorate  and  fresh  acid  should  be  added  from 
time  to  time.  Evaporate  the  solution  to  dryness,  add  hydro- 
chloric acid  and  a  little  water,  and  again  evaporate  to  dryness 
and  heat  to  dehydrate  silica.  Take  up  the  residue  with  50  cc. 
boiling  water  and  2  or  3  cc.  of  cone,  hydrochloric  acid.  Filter 
into  another  porcelain  casserole,  and  repeat  the  evaporation  and 
dehydration  of  silica.  Take  up  with  50  cc.  of  hot  water,  and  2 
or  3  cc.  of  cone,  hydrochloric  acid  as  before,  and  filter.  Unite 
the  two  portions  of  insoluble  matter,  and  reserve  for  further 
treatment. 

Heat  the  filtrate  from  the  above,  and  add,  drop  by  drop,  10 
cc.  of  barium  chloride  solution  until  no  further  precipitate  is 
formed,  and  then  a  few  drops  in  excess.  Allow  to  stand  over- 

97 


98  THE  ANALYSIS  OF  RUBBER 

night,  filter  off  the  barium  sulfate   (which  may  be  discarded), 
wash  well  and  transfer  the  filtrate  to  a  250  cc.  graduated  flask. 

The  insoluble  portions  reserved  above  are  fused  with  sodium 
carbonate  in  a  nickel  crucible,  cooled,  and  the  melt  taken  up 
with  distilled  water.  If  lead,  barium,  or  calcium  sulfates  were 
in  the  insoluble  residue,  they  will  now  appear  as  insoluble  car- 
bonates, while  the  silica,  if  any,  will  be  in  solution.  Filter  off 
the  insoluble  matter,  wash  free  from  alkali,  and  then  dissolve 
the  carbonates  off  the  filter  with  dilute  hydrochloric  acid  and  hot 
water.  Filter  through  the  same  filter  paper,  and  unite  the  filtrate 
with  the  solution  already  in  the  graduated  flask. 

The  filtrate  from  the  separation  of  the  carbonates  contains 
the  silica;  it  should  be  evaporated  to  dryness,  and  the  silica 
dehydrated  and  determined  in  the  usual  way.  The  filter  paper 
from  the  filtration  of  the  lead  and  barium  should  be  ignited,  and 
examined  for  silicates  which  may  not  have  been  attacked  during 
the  fusion. 

The  solutions  united  in  the  graduated  flask  are  now  made  up 
to  the  250  cc.  mark  at  room  temperature;  50  cc.  of  this  solution 
contains  the  fillers  from  0.500  gr.  of  rubber. 

By  this  procedure,  we  have  eliminated  the  sulfuric  acid,  which 
would  prove  so  troublesome  with  lead,  barium,  and  calcium,  but 
in  so  doing,  have  introduced  barium  into  the  solution.  This  is 
of  no  importance,  for  barium  is  usually  determined  on  a  separate 
sample  by  a  short  but  excellent  method. 

Another  clement  is  introduced  through  the  fusion  in  a  nickel 
crucible,  but  nickel  is  not  likely  to  be  found  in  rubber  compounds 
so  that  we  need  merely  eliminate  it  in  its  turn,  and  proceed  with 
our  analysis.  On  account  of  lead,  fusion  in  platinum  is  impos- 
sible, while  fusion  in  iron  would  introduce  serious  complications. 

The  object  in  making  up  a  standard  solution,  is  that  50  cc.  may 
be  taken  for  qualitative  analysis,  and  further  aliquot  portions 
may  be  drawn  for  such  quantitative  tests  as  may  be  desired. 
In  fact,  with  so  few  metals  to  be  determined,1  it  is  frequently 
possible  to  combine  qualitative  and  quantitative  separations  at 
the  same  time. 

If  the  silica  is  less  than  O.o',;,  we  may  assume  that  it  came 

'•  Lead,  iron,  aluminium,  /ino,  calcium,  and  magnesium  are  practically  the  only 
metals  to  ho  determined.  Antimony  and  barium  arc  determined  in  special  tests; 
manganese  will  ho  encountered  where  iron  oxides  arc  present,  hut  Is  not  neces- 
sarily determined. 


MINERAL  ANALYSIS  99 

from  the  talc  used  in  dusting,  and  that  the  silica  pigments,  such 
as  tripoli,  talc,  asbestine,  aluminum  flake,  etc.,  have  not  been  used 
as  fillers. 

The  procedure  for  making  the  qualitative  and  quantitative 
separations  may  be  taken  from  the  standard  text  books,  and 
need  not  be  repeated  here.  A  few  words  of  caution  may  not 
come  amiss. 

In  only  two  cases  has  vermilion  been  found  amongst  many 
hundreds  of  samples  tested;  it  is  too  costly,  and  since  it  is  used 
only  for  its  color,  there  should  be  little  difficulty  in  detecting  this 
substance  from  the  color  of  the  compound. 

Green-colored  samples  should  be  tested  for  arsenic,  not  that  it 
is  likely  to  be  found,  but  merely  to  be  on  the  safe  side.  Arsenic 
colors  should  never  be  used  in  rubber  compounding,  but  it  is  well 
to  see  that  no  one  is  taking  a  chance. 

Copper,  even  in  traces,  should  be  carefully  looked  for,  because 
even  in  small  amounts  its  deteriorating  influence  on  rubber  com- 
pounds is  remarkable. 

Note  whether  or  not  there  is  any  appreciable  quantity  of  mag- 
nesium; a  small  amount  may  be  expected  from  the  talc  used  in 
dusting  stocks  in  the  mill  room,  but  it  should  be  only  a  matter 
of  0.10%  or  so.  More  than  that  requires  a  quantitative  deter- 
mination, owing  to  the  practice  of  using  small  amounts  of  mag- 
nesium oxide  to  activate  organic  accelerators. 

If  the  nitric  acid  solution  of  the  rubber  shows  insoluble 
material,  and  yet  no  silica  is  present,  it  indicates  insoluble  sul- 
fates  of  lead  or  barium,  or  both. 

Black  specks  remaining  after  the  fuming  nitric  acid  treat- 
ment of  the  rubber,  indicates  gas  black  or  lamp  black,  for  which 
a  separate  determination  is  made. 

It  will  be  seen  from  the  description  of  the  mineral  fillers  used 
in  rubber  manufacture,  that  the  following  metals  may  be  found: 
antimony,  lead,  iron,  aluminium,  chromium,  zinc,  calcium, 
barium,  magnesium,  sodium,  and  ammonium  salts.  The  com- 
pounds formed  with  these  metals,  consist  of  oxides,  sulfides, 
sulfites,  sulfates,  carbonates,  and  silicates. 

Oxides.  The  oxides  are  usually  determined  by  difference ;  after 
the  determination  of  the  acid  radicles,  the  excess  of  bases  over 
that  required  to  combine  with  the  acids  is  assumed  to  be  present 
as  oxide. 


100  THE  ANALYSIS  OF  RUBBER 

Sul fides.  Stevens2  determines  the  sulfide  sulfur  as  follows: 
The  apparatus  consists  of  a  Kipp  generator  for  carbon  dioxide, 
a  250  cc.  flask  with  an  inlet  tube  reaching  nearly  to  the  bottom 
of  the  flask,  and  a  ground-in  stopper  carrying  an  outlet  tube 
(an  all-glass  wash  bottle  can  readily  be  adapted  for  the  pur- 
pose), and  connected  to  the  outlet  tube  are  two  absorption 
bottles  containing  lead  acetate  solution.  Place  in  the  flask  10 
cc.  of  cone,  hydrochloric  acid  and  20  to  30  cc.  of  ether,  pass  a 
current  of  carbon  dioxide  through  the  apparatus  until  all  air  is 
removed,  then  remove  the  stopper  and  add  the  sample  (0.1  to  1.0 
gr.,  depending  upon  the  amount  of  sulfide  expected;  where  noth- 
ing is  known  regarding  the  sample,  use  1.0  gr.).  Again  pass  car- 
bon dioxide  through  the  apparatus  for  about  30  minutes,  with  an 
occasional  shaking  of  the  flask.  During  this  period,  the  hydro- 
chloric acid  attacks  the  sulfides,  liberating  hydrogen  sulfide, 
which  is  carried  over  to,  and  absorbed  by  the  lead  acetate  solu- 
tion. The  purpose  of  the  ether  is  to  swell  the  rubber,  and  facili- 
tate the  penetration  of  the  acid  to  all  parts  of  the  sample. 

Heat  gently  to  drive  off  the  ether  and  the  final  traces  of 
hydrogen  sulfide.  Reserve  the  solution  in  the  flask  for  the  deter- 
mination of  sulfate  sulfur.  All  of  the  sulfide  sulfur  is  now  com- 
bined with  the  lead. 

Stevens  determines  the  sulfur  from  this  point  by  adding  acetic 
acid  to  the  lead  acetate  solution  in  order  to  decompose  the  car- 
bonates formed,  the  lead  sulfide  is  filtered  off,  and  washed  free 
from  lead  salts,  transferred  to  a  stoppered  flask,  a  standard 
iodine  solution  added,  and  after  the  reaction  is  complete  the 
excess  of  iodine  is  titrated  with  sodium  thiosulfate.  However, 
any  other  accurate  method  will  answer  the  purpose;  the  lead 
sulfide  may  be  dissolved  in  nitric  acid,  taken  to  fuming  with 
sulfuric  acid,  and  the  lead  sulfate  determined  gravimetrically. 

If  pure  nitrogen  is  available  for  sweeping  out  the  apparatus, 
it  will  be  found  to  be  much  simpler  to  use  sodium  hydroxide  for 
absorbing  the  hydrogen  sulfide;  the  solution  can  be  oxidized 
with  bromine,  and  after  acidification,  the  sulfate  can  be  precipi- 
tated with  barium  chloride;  altogether,  much  simpler,  and 
probably  more  accurate  than  the  lead  acetate  method. 

Sulfide  sulfur,  excepting  antimony  sulfides,  may  also  be  deter- 
mined by  the  ignition  method  of  Schaeffer,  transferring  the  resi- 

2  Analyst,  40,  275-81  (1915). 


MINERAL  ANALYSIS  101 

due  to  a  flask  similar  to  the  one  recommended  by  Stevens,  and 
proceeding  as  directed  by  him  for  driving  over  the  hydrogen 
sulfide.  This  procedure  is  best  for  lead  sulfide;  antimony  and 
mercury  sulfides  sublime  unchanged. 

Sulfites.  Sulfites  and  sulfates  are  transposed  by  heating  with 
sodium  carbonate.  Schaeffer  gives  the  following  method  for 
determining  the  sulfite-sulfur  in  sublimed  blue  lead: 

Boil  1.5  gr.  of  the  sample  with  3  gr.  of  sodium  carbonate; 
allow  to  stand,  filter,  and  wash  thoroughly.  To  the  filtrate,  add 
3  cc.  of  bromine  water,  heat  gently  to  oxidize  the  sodium  sulfite 
to  sulfate,  and  precipitate  the  sulfate  with  barium  chloride.  The 
barium  sulfate  formed  will  contain  both  the  sulfur  present  as 
sulfite,  and  sulfate;  deduct  the  amount  of  sulfur  present  as 
sulfate  from  the  total,  and  the  remainder  is  calculated  to  lead 
sulfite.  (See  determination  of  sulfates  in  the  presence  of  sulfites, 
under  sulfate-sulfur.) 

Sulfates.  Stevens  determines  the  sulfate-sulfur  in  the  residue 
from  the  determination  of  sulfides,  as  follows:  Extract  the  resi- 
due with  hydrochloric  acid  until  no  further  material  can  be  dis- 
solved; unite  the  filtrates,  and  determine  the  sulfur  as  usual. 
It  will  be  noted  that  by  this  means  Stevens  dissolves  out  only 
the  lead  sulfate  and  calcium  sulfate;  barium  sulfate  will  be  only 
slightly  attacked.  This  method  is  therefore  not  applicable  for 
the  determination  of  lithopone,  for  example,  or  in  any  other  case 
where  barium  sulfate  is  present  along  with  some  sulfide. 

We  again  find  Schaeffer's  ignition  process  of  value  in  deter- 
mining the  sulfates.  Boil  the  ignited  residue  with  sodium  car- 
bonate as  directed  under  sulfite-sulfur,  and  filter.  The  function 
of  the  bromine  water  in  the  sulfite  determination  is  to  oxidize 
the  S02  to  S03;  if  instead  of  adding  bromine  water  we  add 
hydrochloric  acid,  and  boil  the  solution,  the  sulfur  dioxide  will 
be  driven  off,  and  we  will  have  remaining  only  the  sulfate-sulfur. 

Carbonates.  Carbonates  can  be  determined  in  an  apparatus 
similar  to  Stevens'  arrangement  for  sulfide-sulfur.  Instead  of  a 
Kipp  generator,  we  use  air  which  has  first  been  passed  through 
a  soda-lime  tower,  to  remove  traces  of  carbon  dioxide.  In  this 
case,  the  absorption  train  consists  of  two  absorption  bottles  con- 
taining cone,  sulfuric  acid  and  potassium  bichromate  (a  and  b) ; 
two  soda-lime  tubes  (c  and  d) ;  and  the  fifth  tube  containing  sul- 
furic acid  and  bichromate  (e).  It  is  vital  in  this  determination 


102  THE  ANALYSIS  OF  RUBBER 

that  tubes  b  and  e  should  be  frequently  refilled,  and  from  the 
same  solution;  only  with  such  precautions  are  we  able  to  main- 
tain the  air  at  the  same  moisture  content  when  it  leaves  e  as 
when  it  entered  c.  Tubes  c,  d,  and  e,  are  weighed  before  and 
after  the  determination;  the  increase  in  weight  is  the  carbon 
dioxide.  Cases  are  known  where  d  actually  lost  weight,  owing 
to  the  fact  that  c  absorbed  all  of  the  C02,  and  the  air  withdrew 
from  d  some  of  its  moisture,  which,  however,  was  reabsorbed  by  e. 

Any  similar  arrangement  will  do  just  as  well,  providing3  the 
gas  used  to  wash  the  apparatus  contains  no  carbon  dioxide,  or 
organic  matter  which  might  be  oxidized  by  the  sulfuric  acid- 
bichromate  mixture;  the  absorption  tubes  are  adequate  for  the 
purpose;  and  the  balance  of  the  moisture  content  of  the  gas  is 
preserved. 

Silicates.  These  have  already  been  separated  by  the  method 
of  getting  the  metals  of  the  fillers  into  solution.  It  is  only  neces- 
sary here  to  repeat  that  all  of  the  silica  is  not  obtained  by  the 
first  dehydration  and  treatment  with  hydrochloric  acid,  no  matter 
how  long  the  process  be  continued;  the  operation  must  be  re- 
peated or  the  error  will  show  up  in  the  determination  of  the  other 
constituents. 

Special  Determinations. 

The  qualitative  and  quantitative  analyses  made  as  prescribed 
in  the  preceding  paragraphs  will  suffice  for  the  determination  of 
most  of  the  metallic  bases,  or  fillers,  but  some  of  these  are  better 
determined  by  special  tests;  amongst  the  mineral  fillers  we  find 
in  this  list  the  antimony  compounds,  lead  chromate,  barium  car- 
bonate, etc.,  and  amongst  the  organic,  carbon  black,  blue,  etc. 

Antimony.  The  principal  trouble  with  antimony  is  getting  it 
into  solution  without  loss.  There  should  be  little  difficulty  once 
this  has  been  accomplished.  Rothe*  treats  the  sample  with 
10-20  cc.  cone,  nitric  acid  and  2  cc.  sulfuric,  and  heats  for  1 
to  2  hours  at  a  moderate  heat;  then  increase  the  heat  until 
all  nitric  acid  is  driven  off  and  the  sulfuric  acid  fumes  strongly. 
More  nitric  acid  is  added,  and  taken  to  fuming,  and  this  opera- 
tion is  repeated  until  the  absence  of  darkening  shows  that  the 

•For  a  more  complete  discussion  on  this  point,  see  Tuttle  and  Yurow.  "The 
Direct  Determination  of  Rubber  by  the  Nitrosite  Method,"  U.  S.  Bureau  of 
Standards  Tech.  Paper,  No.  145  (1919). 

«Chem.  Ztg.  S3,  679  (1909). 


MINERAL  ANALYSIS  103 

organic  matter  is  destroyed.  Dilute  to  100  cc.  and  boil  to  expel 
all  nitric  fumes.  Schmitz  5  takes  from  2  to  4  gr.  of  finely  cut 
rubber  (Frank  and  Marckwald  think  the  quantity  is  too  high,  as 
it  no  doubt  is  for  most  antimony  compounds) ,  and  treats  it  in  a 
Kjeldahl  flask  with  15  cc.  cone,  sulfuric  acid  per  gram  of  rubber. 
One  drop  of  mercury  and  a  small  piece  of  paraffin  (to  prevent 
foaming)  are  introduced.  Heat  until  the  solution  starts  to  clear; 
add  2-4  gr.  of  potassium  sulfate,  and  heat  until  colorless. 
Cool,  dilute  with  water,  add  1  to  2  gr.  of  potassium  bisulfite, 
with  excess  of  tartaric  acid ;  heat  until  no  sulfur  dioxide  remains, 
add  dilute  hydrochloric  acid,  filter,  and  titrate  the  antimony. 
Wagner6  fuses  in  a  porcelain  crucible,  0.5  to  1.0  gr.  of  rubber 
with  5  gr.  of  1-4  sodium  nitrate-potassium  carbonate.  The 
rubber  is  mixed  with  part  of  the  fusion  mixture,  placed  in  the 
crucible,  and  covered  with  the  remainder.  The  heat  must  be 
applied  gradually,  and  if  any  organic  matter  remains,  more 
sodium  nitrate  must  be  added,  and  the  whole  reheated.  Wagner 
claims  good  results,  but  the  method  looks  risky;  the  danger  of 
loss  of  antimony  by  excessive  heating  is  very  great.  When  zinc 
oxide  or  sulfide  are  present,  Frank  and  Marckwald 7  separate 
the  rubber  from  the  fillers  with  xylol ;  otherwise,  they  oxidize  the 
organic  matter  with  cone,  nitric  acid  and  potassium  chlorate, 
finally  evaporating  with  hydrochloric  acid.  If  organic  matter  is 
still  present,  it  must  be  eliminated.  The  antimony  is  precipi- 
tated as  sulfide,  and  weighed  as  such.  Collier,  Levin  and 
Scherrer8  take  advantage  of  the  simultaneous  determination  of 
the  fillers  by  the  cymene  method  to  determine  the  antimony  after 
the  rubber  has  been  dissolved  out.  Their  method  is  as  follows: 

Extract  0.500  gr.  of  the  sample  with  acetone  for  8  hours, 
and  with  chloroform  for  4  hours.  Dry  the  residue  in  a  vacuum 
desiccator,  transfer  to  a  300  cc.  lipped  assay  flask,  add  25  cc.  of 
cymene,  and  heat  on  an  electric  hot  plate  at  130-140C  until  the 
rubber  is  dissolved.  Cool  the  flask,  dilute  with  250  cc.  of  petro- 
leum ether,  and  allow  to  stand  overnight.  Filter  by  decantation 
through  a  tight  Gooch  pad  of  asbestos,  previously  washed  with 
alkali,  cone,  hydrochloric  acid,  and  water,  and  dried.  Wash  by 
decantation  with  petroleum  ether  until  the  filtrate  is  colorless. 

8Gummi  Ztg.   25,  1928-30    (1911). 

•Chem.  Ztg.  SO,  638  (1906)  ;  J.  Soc.  Chem.  Ind.  25,  583  (1906). 

'Gummi  Ztg.  23,  1046  (1909). 

•Rubber  Age,  8,  104-5   (1920). 


104  THE  ANALYSIS  OF  RUBBER 

Add  30  cc.  of  cone,  hydrochloric  acid  to  the  assay  flask,  and 
shake  until  all  of  the  antimony  sulfide  has  gone  into  solution; 
filter  slowly  through  the  Gooch,  using  gentle  suction.  Wash 
thoroughly,  and  dilute  the  filtrate  to  250  cc.  with  hot  distilled 
water,  pass  in  hydrogen  sulfide  until  the  antimony  has  been  com- 
pletely precipitated. 

After  the  solution  of  the  antimony  has  been  effected,  it  may 
be  determined  by  any  of  the  well  known  methods.  Wagner,  and 
Frank  and  Marckwald  weigh  as  sulfide,  Schmitz  recommends 
titration,  as  do  Collier,  Levin  and  Scherrer.  The  methods  recom- 
mended by  the  last  named  are  as  follows: 

Filter  off  the  antimony  sulfide,  wash  with  hydrogen  sulfide 
water,  and  transfer  the  precipitate  to  the  filter  paper.  Place  20 
cc.  of  concentrated  hydrochloric  acid  in  the  beaker,  and  set  aside 
temporarily.  Transfer  the  antimony  sulfide  and  the  filter  paper 
to  a  Kjeldahl  flask,  add  12-15  cc.  of  concentrated  sulfuric  acid 
and  5  gr.  of  potassium  sulfate,  place  a  funnel  in  the  neck  of  the 
flask,  and  heat  until  the  solution  is  colorless.  Wash  the  funnel, 
and  dilute  the  solution  to  about  100  cc.  with  water,  add  1-2  gr. 
of  sodium  sulfite,  transfer  the  hydrochloric  acid  in  the  beaker 
in  which  the  antimony  sulfide  was  precipitated  to  the  Kjeldahl 
flask,  and  boil  until  the  sulfur  dioxide  is  all  driven  out.  Dilute 
to  250-275  cc.  with  water,  cool  to  10-15C,  and  titrate  with  per- 
manganate until  a  faint  pink  color  is  obtained. 

Instead  of  filtering  the  antimony  on  filter  paper,  it  may  be 
filtered  on  a  Witt  plate  and  asbestos.  Transfer  the  plate,  pad 
and  precipitate  to  an  Erlenmeyer  flask;  remove  any  antimony 
sulfide  adhering  to  the  beaker  or  funnel  with  hydrochloric  acid. 
Wash  the  beaker  and  funnel  with  hot  distilled  water,  dilute  the 
solution  to  250-275  cc.,  add  12  cc.  of  concentrated  sulfuric  acid, 
boil  the  solution  until  no  trace  of  hydrogen  sulfide  is  obtained 
with  lead  acetate  paper,  cool  to  10-15C,  and  titrate  with  stand- 
ard permanganate  solution. 

Barium  Salts.  Ignite  a  1  gr.  sample  in  a  porcelain  crucible, 
cool,  add  3  to  5  drops  of  nitric  acid  and  1  cc.  of  water,  and  stir 
into  a  paste,  add  5  gr.  of  1-1  potassium  nitrate-sodium  carbonate, 
dry  on  the  hot  plate  or  steam  bath,  fuse  until  the  melt  is  soft 
or  pasty ;  allow  it  to  cool,  extract  with  hot  water,  and  wash  with 
hot  water  containing  a  little  sodium  carbonate.  Dissolve  the  in- 
soluble carbonates  in  hydrochloric  acid,  and  wash  the  filter  paper 


MINERAL  ANALYSIS  105 

thoroughly.  Nearly  neutralize  the  filtrate  with  sodium  carbon- 
ate, and  pass  hydrogen  sulfide  through  the  solution  until  the  lead 
is  entirely  precipitated.  Filter,  heat  the  filtrate  to  boiling,  and 
add  10  cc.  of  10%  sulfuric  acid;  allow  the  precipitate  to  stand 
overnight,  and  determine  the  barium  sulfate  as  usual. 

The  only  troublesome  element  is  lead,  and  it  may  be  com- 
pletely eliminated.  Check  determinations  of  0.10%  of  the 
barium  sulfate  present  may  easily  be  obtained. 

In  some  specifications,  a  maximum  limit  is  placed  on  the  total 
sulfur,  but  barytes  is  a  permissible  filler,  without  having  the 
sulfur  which  it  contains  count  as  part  of  the  total  sulfur.  In 
such  cases,  the  determination  of  barytes  is  obligatory ;  if  made  by 
this  method,  the  error  in  the  total  sulfur  caused  by  the  correc- 
tion will  not  exceed  0.02%. 

Barium  Carbonate*  Place  1  gr.  of  the  sample  in  a  porcelain 
boat,  and  ignite  in  an  atmosphere  of  carbon  dioxide  as  described 
by  Schaeffer.10  After  ignition,  and  when  the  ash  is  at  room  tem- 
perature, remove  the  boat,  grind  the  ash  to  a  fine  powder  in  an 
agate  mortar,  transfer  to  a  250  cc.  beaker,  cover  with  5-10  gr.  of 
ammonium  carbonate,  15-20  cc.  of  strong  ammonia,  and  50  cc.  of 
distilled  water.  Ammonium  carbonate  transposes  lead  sulfate 
into  lead  carbonate,  but  is  practically  without  action  on  barium 
sulfate.  Boil  the  mixture  for  15  to  30  minutes,  filter,  and  wash 
the  precipitate  thoroughly  to  remove  all  soluble  sulfates.  Wash 
the  residue  on  the  filter  paper  back  into  the  original  beaker  with 
distilled  water,  add  10  cc.  glacial  acetic  acid,  and  sufficient  water 
to  make  the  volume  up  to  100  cc.  Heat  to  boiling,  and  filter 
through  the  same  filter  paper  as  before.  Lead,  barium  calcium 
and  zinc  carbonates  pass  into  solution,  whereas  lead  sulfide  and 
barium  sulfate  are  not  attacked.  Pass  hydrogen  sulfide  into  the 
filtrate,  filter  off  the  lead  sulfide,  heat  the  filtrate  to  boiling,  and 

8  The  reason  for  working  out  a  method  for  determining  barium  carbonate  is 
not  without  interest.  In  material  made  under  specifications,  some  manufac- 
turers evidently  desired  to  use  compounds  which  contained  more  than  the  pre- 
scribed amount  of  sulfur.  Realizing  that  the  specifications  exempted  the  sulfur 
in  the  barytes  from  counting  in  the  total  sulfur,  and  knowing  that  the  barium 
sulfate  was  being  estimated  from  the  amount  of  barium  found  by  analysis,  they 
felt  that  by  adding  barium  carbonate,  they  would  receive  credit  for  sulfur 
equal  to  the  barium  in  the  carbonate,  and  thus  bring  the  total  within  the 
specification  limit.  The  trick  was  first  discovered  whpn.  after  correction  for  tin- 
sulfur  supposed  to  be  present  in  combination  with  the  barium,  the  total  sulfur 
was  actually  less  than  the  free  sulfur. 

10  Cf.  page  91. 


106  THE  ANALYSIS  OF  RUBBER 

precipitate  the  barium  with  10  cc.  of  10%  sulfuric  acid.  Allow 
to  stand  overnight,  and  determine  the  barium  sulfate  as  usual. 

If  barium  sulfate  and  no  carbonate  is  present,  a  small  amount 
of  precipitate  will  be  found,  showing  a  slight  solubility  of  the 
barium  sulfate,  or  else  reduction  of  the  sulfate  to  sulfide.  The 
amount  will  usually  be  less  than  1%  of  the  amount  of  barium 
sulfate  present.  In  a  mixture  of  the  two,  the  carbonate  will  run 
somewhat  high,  for  the  same  reasons,  but  with  proper  attention 
to  details  the  results  will  be  quite  sufficient  for  every  purpose. 

Gas  Black  or  Lamp  Black.  Chemical  analysis  alone  will  tell 
nothing  as  to  whether  gas  black  or  lamp  black  has  been  used. 
Even  the  microscope  is,  as  yet,  of  little  value  in  distinguishing 
between  the  two,  and  the  only  thing  remaining  for  us  to  do  is  to 
determine  the  total  carbon,  and  assume  from  the  physical  prop- 
erties of  the  article,  whether  or  not  the  black  is  gas  black  or 
lamp  black. 

The  free  carbon  is  determined  as  follows:  1X 

Extract  0.5  gr.  of  rubber  for  8  hours  with  a  mixture  by  volume 
of  68%  chloroform  and  32%  acetone.  Transfer  the  sample  to  a 
250  cc.  beaker,  and  heat  until  it  no  longer  smells  of  chloroform. 
Add  a  few  cc.  of  hot  cone,  nitric  acid,  and  allow  to  stand  in  the 
cold  for  about  10  minutes.  Add  50  cc.  more  of  hot  cone,  nitric 
acid,  taking  care  to  wash  down  the  sides  of  the  beaker;  heat  on 
the  steam  bath  for  at  least  an  hour.  While  hot,  decant  the  liquid 
through  a  Gooch  containing  a  thick  pad  of  asbestos,  taking  care 
to  keep  the  insoluble  material  completely  in  the  beaker.  Wash 
with  hot  nitric  acid,  and  suck  dry.  Empty  the  filter  flask.  Wash 
the  insoluble  residue  with  acetone,  and  then  with  a  mixture  of 
equal  parts  of  acetone  and  chloroform,  until  the  filtrate  is  color- 
less. The  insoluble  matter,  which  has  been  carefully  retained  in 
the  beaker,  is  digested  on  the  steam  bath  for  30  minutes  with  35 
cc.  of  a  25%  solution  of  sodium  hydroxide.  Dilute  to  60  cc.  with 
hot  water,  filter  the  solution,  and  wash  with  a  hot  15%  solution 
of  sodium  hydroxide.  Test  for  the  presence  of  lead  by  running 
some  warm  ammonium  acetate  solution  containing  an  excess  of 
the  hydroxide  through  the  pad  into  sodium  chromate ;  if  a  yellow 
precipitate  is  obtained,  the  pad  must  be  washed  until  the  wash- 
ings no  longer  give  a  precipitate  with  the  sodium  chromate 

n  Smith  and  Epstein,  U.  S.  Bureau  of  Standards  Tech.  Paper,  No.  136 ;  J.  Ind. 
Eng.  Chem.  11,  33-6  (1919). 


MINERAL  ANALYSIS  107 

solution.  Next  wash  the  residue  a  few  times  with  hot  cone, 
hydrochloric  acid,  and  finally  with  warm  5%  hydrochloric  acid. 
Remove  the  crucible  from  the  funnel,  taking  care  that  the  outside 
is  perfectly  clean,  and  dry  in  an  air  bath  at  150C  to  constant 
weight.  Burn  off  the  carbon  at  a  dull  red  heat,  cool  and  reweigh ; 
the  difference  in  weight  is  approximately  105%  of  the  carbon 
originally  present  in  the  form  of  lampblack  or  gas  black. 

Several  points  must  be  carefully  watched  during  this  pro- 
cedure: the  acetone  and  hot  nitric  acid  must  not  be  brought  to- 
geth,er,  since  they  react  with  considerable  violence.  Again,  care 
must  be  used  in  the  alkali  washing  to  avoid  carrying  through 
the  filter  some  of  the  gas  black;  the  pad  must  be  unusually  thick 
and  free  from  channels.  This  is  one  of  the  principal  reasons  for 
keeping  the  fillers  in  the  beaker  until  the  last  moment. 

The  published  data  of  Smith  and  Epstein  show  that  the  loss  in 
weight  on  ignition  is  about  5%  higher  than  the  carbon  actually 
present;  hence  the  factor  105.  The  5%  is  probably  organic 
matter  not  removed  by  the  preliminary  steps  of  the  method. 
Mineral  rubber  has  no  effect  on  the  determination.  Calcium  sul- 
fate,  if  retained  with  the  fillers,  would  be  reduced  during  the 
ignition  of  the  carbon,  and  would  give  high  results  for  the  latter. 
Quite  apart  from  the  point  raised  by  Smith  and  Epstein  that 
calcium  sulfate  is  rarely  found  in  rubber  compounds,  usually  only 
when  associated  with  antimony,  the  treatment  with  strong  acids, 
and  boiling,  would  suffice  to  dissolve  out  a  considerable  quantity 
of  calcium  sulfate,  which  is  quite  soluble  in  hot  nitric  or  hydro- 
chloric acid  solutions. 

Red  Lead.  The  peroxide  of  lead  contained  in  red  lead  is  not  a 
particularly  desirable  constituent  for  rubber  compounds,  and 
some  specifications,  notably  those  for  30  or  40%  Para  insulation, 
forbid  its  use.  The  Joint  Rubber  Insulation  Committee 12  gives 
the  following  test  for  red  lead:  Dissolve  a  1  gr.  sample,  pre- 
viously extracted  with  acetone,  in  xylol;  when  the  rubber  has 
been  completely  dissolved,  filter  through  a  Gooch  crucible,  wash- 
ing thoroughly  with  benzol,  alcohol  and  acetone.  Transfer  the 
Gooch  pad  to  a  distilling  flask,  add  hydrochloric  acid,  and  distil 
over  the  chlorine  into  a  potassium  iodide-starch  solution.  If 
more  than  0.1  cc.  of  N/10  sodium  thiosulfate  is  required  to  titrate 
the  iodine  liberated,  red  lead  may  be  assumed  to  be  present. 

UJ.  Ind.  Eng.  Chem.  6,  75-82  1914). 


108  THE  AXALYMS  OF  RUBBER 

This  mi-thud  was  suggested  for  insulation  compounds,  and,  as 
Jar  as  it  has  been  tested,  has  given  satisfactory  results.  The 
method  depends  upon  the  liberation  of  chlorine  by  the  action  of 
the  peroxide  on  the  hydrochloric  acid.  Some  off-color  litharge 
samples  have  given  positive  tests  under  this  method;  which  is 
what  we  might  expect,  since  these  lots  contain  a  greater  amount 
of  peroxide  than  they  should,  and  yet  not  enough  to  be  classed 
as  red  lead.  They  are  really  mixtures  of  red  lead  and  litharge, 
and  should  be  so  treated. 

Chromates.  such  as  chrome  yellow,  will  give  this  reaction,  but 
they  should  cause  no  confusion,  since  the  color  of  the  sample  will 
usually  tell  whether  chromates  are  present.  It  would  be  unusual 
indeed  to  have  both  chromates  and  lead  peroxide  present  in  the 
same  sample. 

Chromates.  While  chromium  is  not  a  frequent  constituent  of 
rubber  goods,  it  is  a  possibility,  and  should  be  determined.  There 
is  considerable  analogy  between  the  analyses  of  the  pigments  in 
printing  inks,  and  those  in  rubber  compounds,  and  the  following 
method,  originally  used  in  the  analysis  of  printing  inks,  should 
be  equally  available  for  rubber  compounds. 

Fuse  0.500  gr.  of  rubber  with  equal  parts  of  sodium  peroxide 
and  potassium  carbonate,  using  a  nickel  crucible.  The  heating 
must  proceed  cautiously  until  the  organic  matter  is  destroyed, 
after  which  the  melt  can  be  heated  strongly  for  10  or  15  minutes. 
Cool,  extract  with  water,  and  filter.  The  chromium  is  in  the  fil- 
trate as  chromate.  Pass  carbon  dioxide  through  the  filtrate,  and 
heat  on  the  steam  bath,  in  order  to  precipitate  any  lead  which 
may  be  held  up  by  the  caustic  alkali;  filter  if  necessary.  Cool, 
acidify  strongly  with  hydrochloric  acid,  add  potassium  iodide 
and  starch  solution,  and  titrate  with  standard  N/10  sodium 
thiosulfate  to  a  colorless  solution.  The  solution  may  be  stand- 
ardized against  potassium  bichromate,  and  the  chromium  calcu- 
lated to  CrO:{,  in  which  condition  it  no  doubt  exists  in  the  com- 
pound. 

This  method  has  been  found  simple  and  accurate  in  the  pres- 
ence of  lead,  manganese,  clay,  and  other  fillers  likely  to  be 
present  in  printing  inks,  and  should  be  fully  as  satisfactory  for 
rubber  goods. 

Glue.  Make  a  qualitative  test  as  follows:  Digest  1  gr.  in 
cresol,  or  xylol  (any  solvent  for  rubber  which  does  not  attack 


MINERAL  ANALYSIS  109 

glue  will  do  just  as  well)  until  the  rubber  is  decomposed.  Dilute 
with  petroleum  ether,  and  filter  through  filter  paper.  Wash  the 
residue  with  alcohol,  and  after  allowing  the  alcohol  to  evaporate 
wash  the  residue  back  into  a  beaker,  cover  with  water,  and  boil. 
Filter  off  the  insoluble,  and  test  the  filtrate  for  glue  with  a  solu- 
tion of  tannic  acid.  Traces  of  glue  will  give  only  a  milky  cloudi- 
ness, but  with  large  quantities  a  heavy  precipitate  is  thrown 
down.18 

The  quantitative  determination  of  glue  is  based  on  the  deter- 
mination of  nitrogen  by  the  Kjeldahl  method.  This  procedure 
assumes  that  the  principal  source  of  nitrogen,  the  organic  accel- 
erators, will  be  removed  during  the  acetone  extract.  The  U.  S. 
Bureau  of  Standards  extracts  with  the  mixed  solvents,  68%  by 
volume  of  chloroform,  and  32%  of  acetone.  From  this  point  on, 
the  procedures  are  alike :  the  dried  sample  is  heated  with  sulfuric 
acid,  potassium  or  sodium  sulfate  and  a  small  amount  of  copper 
sulfate,  the  clear  solution  is  diluted,  made  alkaline,  and  the 
ammonia  distilled  into  a  standard  solution  of  N/10  sulfuric  acid. 

Practically  every  one  is  agreed  that  the  Kjeldahl  method  is 
the  most  satisfactory  means  of  approach  in  the  quantitative  de- 
termination of  glue,  but  differences  arise  as  to  the  factor  by  which 
to  calculate  from  nitrogen  to  glue.  The  Bureau  of  Standards 
uses  5.56;  others  prefer  the  factor  of  6.25.  Since  glue  is  not  a 
pure  chemical  substance,  we  are  bound  to  have  differences  of 
opinion,  but  the  weight  of  evidence  seems  to  lean  towards  the 
higher  factor.  The  collagens  have  17.9%  of  nitrogen,  and  even 
assuming  that  in  glue  we  have  reasonably  pure  collagens,  we  must 
take  into  consideration  the  water  which  is  always  present,  and 
which  will  average  about  10%,  hence,  in  the  collagens,  5.56 
would  be  the  correct  figure,  and  calculating  that  this  is  only  90% 
of  the  whole,  we  get  6.18  as  the  corrected  figure. 

Another  variable  will  be  the  amount  of  nitrogen  in  the  in- 
soluble matter  in  the  rubber,  which,  as  we  have  already  discussed, 
may  run  from  2  to  6%  of  the  rubber,  and  may  contain  from  10 
to  18%  of  nitrogen. 

In  view  of  the  above  facts,  it  is  obvious  that  any  factor  will 

18  A  much  shorter  method  of  testing  qualitatively  for  glue  is  as  follows : 
Heat  5  to  10  gr.  finely  divided  sample  of  rubber  with  25  cc.  of  water  for  2  to  4 
hours  ;  decant,  and  test  for  glue  with  2  or  3  cc.  of  a  solution  of  tannic  acid. 
This  test  is  not  as  safe  as  the  one  given  above ;  glue  to  the  extent  of  2  or  3% 
may  be  easily  overlooked,  and  hence  the  method  is  not  recommended. 


110  THE  ANALYSIS  OF  RUBBER 

at  best  give  only  an  approximation  of  the  truth,  but,  even  so,  it 
is  believed  that  better  results  on  the  average  will  come  from 
the  use  of  the  factor  6.25,  and  which  we  recommend. 

Ground  Organic  Wastes.  In  a  mixture  of  rubber  and  wastes, 
containing  such  materials  as  leather,  cork,  wool,  silk,  cotton  or 
other  vegetable  fibre,  etc.,  the  separate  determination  of  these 
wastes  is  usually  of  no  consequence,  and  a  direct  determination 
of  the  rubber  by  the  nitrosite  method  will  give  practically  all  the 
information  that  one  really  needs.  Some  of  the  solvents,  such  as 
xylol,  cymene,  and  possibly  others,  will  determine  the  rubber 
accurately  enough  in  the  presence  of  such  materials. 

There  are  occasions  when  we  may  be  called  upon  to  determine 
cotton,  as  for  example,  in  balloon  fabrics,  where  it  is  difficult  to 
separate  the  rubber  from  the  cotton,  etc.  For  this  purpose,  the 
method  of  Epstein  and  Moore 14  will  suffice: 

Treat  a  0.500  gr.  sample  of  the  rubber  with  25  cc.  of  freshly 
distilled  cresol  (b.p.1980)  for  4  hours  at  165C.  Cool,  add 
200  cc.  of  petroleum  ether  very  slowly,  and  with  constant  agii 
tion.  Filter  through  a  Gooch,  and  wash  with  petroleum  ethe 
then  with  hot  benzene,  and  finally  with  acetone.  Add  hot  1 
hydrochloric  acid,  and  transfer  the  contents  of  the  flask  to 
Gooch,  and  wash  at  least  ten  times  with  hot  acid.  Wash  free 
chlorides,  and  then  with  acetone  until  the  filtrate  is  colorlc 
Wash  with  a  mixture  of  equal  parts  of  acetone  and  carl 
bisulfide.  Wash  with  alcohol,  and  dry  for  1%  hours  at  1( 
Transfer  the  asbestos  pad  and  fillers  to  a  weighing  bottle, 
for  about  10  minutes  further,  cool  and  weigh. 

Transfer  the  contents  of  the  weighing  bottle  to  a  50  cc.  bej 
and  pour  over  it  15  cc.  of  acetic  anhydride  and  1  to  2  cc.  of 
sulfuric  acid,  and  digest  on  the  steam  bath  for  one  hour, 
dilute  with  25  cc.  90%  acetic  acid,  and  filter  through  a  weighe 
Gooch.    Wash  with  hot  90%  acetic  acid  until  the  filtrate  is  coloi 
less,  and  then  four  times  more.  Wash  about  "  tin,  ^  with  acetoi 
remove  the  crucible  from  the  funnel,  and  dry  to  cc^.'unt  woij 
at  150C.     The  cellulose  has  been  dissolved  out,  and  the  nsuj 
calculations  are  made. 

Sponge  Rubber.     One  of  the  interesting  points  in  connection 
with  the  analysis  of  sponge  rubber  is  to  determine  the  substance 

14  U.   S.  Bureau  of   Standards  Tech.  Paper,  154;   The  Rubber  Age,  6,  289-93 
(1920). 


MINERAL  ANALYSIS  111 

used  to  produce  porosity.  Organic  liquids,  if  used,  will  have  been 
dissipated  by  the  time  the  sample  reaches  the  analyst.  If  either 
ammonium  carbonate  or  sodium  carbonate  has  been  used  suf- 
ficient material  will  usually  remain  to  give  a  qualitative  test, 
although  a  quantitative  determination  is  out  of  the  question. 

Grind  the  sample  into  small  particles,  being  particularly  care- 
ful to  avoid  heating.  Digest  10  gr.  of  the  sample  in  25  cc.  of 
water  for  one  hour,  and  filter.  Divide  into  two  portions;  into 
the  first,  add  10  cc.  of  20%  caustic  soda,  and  note  any  odor  of 
ammonia  which  may  escape.  A  positive  test  indicates  ammo- 
nium carbonate.  A  more  delicate  test  may  be  made  by  adding 
a  little  hydrochloric  acid,  and  evaporating  to  dryness,  and  treat- 
ing the  dried  residue  with  a  small  amount  of  strong  alkali.  Evap- 
orate the  second  portion  of  the  extract  to  dryness,  take  up  with 
25  cc.  of  water,  and  add  a  few  drops  of  methyl  orange.  Titrate 
with  N/10  hydrochloric  acid;  any  appreciable  quantity  of  alka- 
line carbonate,  in  the  absence  of  ammonia,  will  be  a  fair  indica- 
tion that  sodium  bicarbonate  was  used.  In  case  ammonium  car- 
bonate was  used,  the  residue  from  the  second  filtrate  should  be 
heated  strongly  to  remove  the  ammonia,  and  thus  determine 
whether  both  substances  were  used. 

Negative  tests  for  both  ammonium  carbonate  and  sodium  bi- 
carbonate may  be  taken  to  indicate  that  organic  liquids  have 
been  employed. 

Specific  Gravity. 

Rubber  Compounds.  For  ordinary  rough  work,  where  great 
accuracy  is  not  necessary,  and  when  pieces  of  from  2  to  5  gr.  are 
available,  Young's  gravitometer  is  a  rapid  and  convenient  instru- 
ment. When  the  bearings  are  clean,  and  the  instrument  in  good 
working  order,  the  results  are  usually  with  0.02,  plus  or  minus 
and  are  frequently  only  half  that. 

For  greater  accuracy,  the  pycnometer  is  the  best  thing  to  use. 
Weigh  out  about  5  gr.  in  small  strips,  place  them  in  the  pycnom- 
eter bottle,  and  fill  with  distilled  water  to  the  mark,  being 
careful  that  no  bubbles  adhere  to  the  rubber,  and  then  weigh. 
Knowing  the  weight  of  the  bottle  filled  with  water,  the  weight 
of  water  displaced  by  the  rubber  is  easily  calculated,  and  from 
this,  the  specific  gravity  of  the  rubber.  Ordinarily  the  specific 
gravity  is  expressed  to  two  decimal  places,  but  even  without 


112  THE  ANALYSIS  OF  RUBBER 

bringing  the  pycnometer  to  constant  temperature,  the  calculations 
may  be  made  to  the  third  decimal. 

It  has  been  found  convenient,  both  in  using  Young's  gravi- 
tometer  and  the  pycnometer,  to  wet  the  rubber  with  a  soap  solu- 
tion, brushing  it  on  with  a  camel's  hair  brush,  and  then  rinsing 
the  rubber  with  distilled  water.  It  eliminates  the  risk  of  air 
bubbles,  and  does  not  affect  the  accuracy  of  the  determination. 

Pigments  and  Fillers.  Pigments  in  lumps  may  be  handled  as 
in  the  case  of  rubber  compounds;  the  pycnometer  is  probably 
better  for  the  purpose. 

Oils  are  determined  with  the  Westphal  balance,  or,  for  quicker 
and  less  accurate  work,  a  hydrometer  will  do. 

For  powders,  or  small  particles,  the  pycnometer  is  required. 
The  liquid  chosen  must  be  such  as  to  have  no  effect  on  the  pig- 
ment being  tested.  For  many  of  them,  water  will  answer,  but 
where  this  is  impossible,  any  other  liquid  will  do  just  as  well, 
providing  it  does  not  react  with,  or  dissolve  the  pigment.  With 
liquids  %  other  than  water,  the  coefficient  of  expansion  may  be 
such  as  to  make  it  imperative  to  hold  to  a  standard  temperature 
of  say  25C,  the  specific  gravity  being  referred  to  water  at  that 
temperature. 

Weigh  out  5  gr.  of  the  pigment,  transfer  to  a  pycnometer,  and 
fill  the  latter  about  two-thirds  full.  Boil  the  liquid  for  10  to  15 
minutes,  and  then  place  under  a  vacuum  bell  jar.  When  the  air 
has  been  entirely  removed  from  the  sample,  cool  to  room  tem- 
perature (or  to  a  standard  temperature  of  25C),  fill  up  to  the 
mark,  and  weigh.  When  a  liquid  other  than  water  is  used,  deter- 
mine its  specific  gravity  as  referred  to  water  at  25C,  and  use 
this  to  calculate  the  gravity  of  the  pigment. 

Reclaimed  Rubber.  One  of  the  important  values  connected 
with  reclaimed  rubber  is  its  gravity,  and  yet  it  is  frequently  so 
porous  that  ordinary  methods  fail  to  secure  accurate  results. 
In  thin  sheets,  and  with  boiling  water,  fair  results  may  be  ob- 
tained. When  a  small  mixing  mill  has  been  available,  the 
following  scheme  has  been  found  eminently  satisfactory: 

Mix  450  grams  of  reclaimed  rubber  and  50  grams  of  sulfur, 
until  thoroughly  and  evenly  mixed.  The  total  weight  of  the 
batch  after  mixing  should  be  within  1  gr.  of  500.  Vulcanize  a 
small  strip  from  the  mix,  and  from  this  strip  determine  the 
specific  gravity  of  the  mixture,  Calculate  the  specific  gravity  of 


MINERAL  ANALYSIS  113 

the  reclaimed  rubber,  taking  the  specific  gravity  of  sulfur  as  2.0. 
If  the  specific  gravity  of  the  mixture  is  a,  and  the  specific  gravity 
of  the  reclaim  x,  the  calculation  is  as  follows: 

lOOa  —  20.00 

•v  

90 

For  example,  if  the  gravity  of  a  mixture  is  1.370,  the  calcula- 
tion would  be: 

_100     X     1.370     -     20.00    _  117.00 
90.  "  90.00 

x=  1.30 

A  chart  can  be  drawn,  so  that  given  the  specific  gravity  of  a 
mixture  that  of  the  reclaimed  can  be  read  off  directly.  A  differ- 
ent mixture  of  reclaimed  rubber  and  sulfur  may  be  employed, 
making  the  necessary  alterations  in  the  formula,  the  only  requi- 
site being  that  there  should  be  sufficient  sulfur  for  vulcanization. 


Chapter  XI. 
Microsectioning  and  Microphotography. 

Microphotographs  of  rubber  goods  have  been  known  for  a 
number  of  years,  Weber  showing  some  excellent  photographs  of 
hard  and  soft  rubber  goods  in  his  book  on  India  Rubber.  Re- 
cently, there  has  been  considerable  attention  paid  to  the  use 
of  the  microscope  in  mineral  analysis  of  small  amounts  of 
materials,  and  in  the  examination  of  commercial  materials,  mix- 
tures, etc.  It  has  been  realized  that  the  chemical  analysis  does 
not  give  the  last  word,  and  that  frequently  the  difference  in 
the  properties  of  two  materials  may  be  a  matter  of  their  physical 
state,  rather  than  their  average  chemical  composition.  In  the 
rubber  industry,  many  laboratories  have  been  working  along 
the  lines  of  preparing  sections  of  rubber  compounds  thin  enough 
to  be  examined  under  transmitted  light,  instead  of  reflected  light, 
as  had  been  so  largely  the  practice.  The  problem  very  quickly 
narrowed  itself  down  to  a  question  of  mechanical  manipulation, 
for  even  the  crude  sections  first  prepared  showed  that  the  pro- 
cedure was  feasible,  and  that  information  could  be  obtained  not 
only  regarding  composition,  but  even  the  properties  of  rubber 
compounds,  if  the  proper  sections  could  be  prepared. 

The  microsectioning  has  largely  been  done  with  the  Spencer 
microtome,  which  seems  adequate  for  the  purpose.  The  main 
difficulty  has  been  to  so  stiffen  the  rubber  compound  that  it 
would  have  no  motion  when  being  cut.  Freezing  was  resorted 
to,  the  earliest  attempts  employing  the  expansion  of  carbon 
dioxide  directly  on  the  stage  of  the  microtome,  or  surrounding 
the  specimen  to  be  cut  with  solid  carbon  dioxide.  Further 
stiffening  of  the  rubber  was  obtained  by  imbedding  it  in  such 
materials  as  starch  paste,  water-glycerine  solutions,  paraffin,  etc. 
The  best  results  are  obtained  with  material  which  docs  not 
become  brittle1  at  the  low  temperatures  employed.  Even  carbon 
dioxide  cooling  was  found  to  bo  insufficient  for  the  purpose,  and 
the  use  of  liquid  air  was  resorted  to,  with  eminently  satisfactory 

114 


MICROSECTIONING  AND  MICROPHOTOGRAPHY    115 

results.  Sections  thinner  than  I/*  are  now  being  prepared,  a  great 
deal  of  work  has  been  started,  and  we  are  beginning  to  see  the 
fruits  of  this  work. 

Liquid  air  is  probably  not  available  for  many  laboratories, 
but  in  such  cases  the  use  of  carbon  dioxide  alone  will  be  found 
to  give  results  well  worth  the  effort,  even  though  better  could 
be  obtained  with  the  cooling  effected  by  the  liquid  air. 

Perhaps  one  of  the  most  interesting  points  brought  out  by  this 
new  phase  of  rubber  testing  came  to  light  at  the  meeting  of 
the  American  Chemical  Society  at  Rochester,  in  April,  1921. 
Schippel x  had  previously  shown  by  experiment  that  compounded 
and  vulcanized  rubbers  showed  an  increase  in  volume  on  stretch- 
ing, and  his  explanation  was  that  vacu  were  formed  around  the 
mineral  particles,  caused  by  the  rubber  being  pulled  away  from 
the  surface  of  the  pigment.  Green2  exhibited  some  microphoto- 
graphs  of  sections  of  rubber  under  strain,  wherein  the  vacu 
caused  by  the  rubber  leaving  the  surface  of  the  pigment  were 
clearly  visible.  Still  more  important  was  the  evident  fact  that 
only  the  larger  or  coarser  particles  showed  this  phenomenon. 
The  mechanism  of  tearing,  rapid  wear,  etc.,  when  coarse  pig- 
ments are  used,  was  quite  apparent.  Green's  work  reflects  credit 
on  the  soundness  of  SchippeFs  reasoning. 

The  work  of  Breyer  and  his  coworkers  Ruby,  Depew,  and 
Green,  and  of  I.  C.  Diner,  should  shortly  put  us  in  a  position 
where  we  can  take  a  piece  of  rubber  and  at  least  qualitatively 
tell  what  pigments  are  present.  It  is  too  much  to  expect  any- 
thing in  the  quantitative  line,  especially  when  one  considers  the 
extremely  small  area  covered  by  these  microphotographs,  and 
the  difficulty  of  securing  even  mixing  of  a  plastic  such  as  rubber 
with  dry  fillers.  We  know  that  we  have  variations  in  compo- 
sition from  one  part  of  a  batch  to  another;  and  this  variation 
must  be  very  much  greater  when  the  sample  under  observation 
weighs  less  than  a  milligram.  It  is  quite  within  the  range  of 
probability  that  we  shall,  by  careful  sectioning,  be  able  to  tell 
whether  we  are  dealing  with  carbon  black  or  lamp  black;  and 
particularly  identify  such  substances  as  Tripoli,  aluminum  flake, 
talc,  asbestine,  etc.,  in  mixtures  of  two  or  more,  under  which 

»J.  Ind.  Bng.  Chem.  12,  33-7   (1920). 

*  Henry  Green.  "Volume  increase  of  compounded  rubber  under  strain,"  Rub- 
ber Division,  American  Chemical  Society,  Rochester,  April,  1921. 


116  THE  ANALYSIS  OF  RUBBER 

conditions  the  identification  by  chemical  or  mechanical  means  is 
practically  impossible. 

The  general  scheme  for  the  examination  of  microsections  3  deals 
with  (a)  reflected  light;  (6)  transmitted  light;  (c)  polarized  light. 
With  reflected  light,  we  use  not  only  vertical,  but  oblique  rays, 
so  as  to  get  some  idea  of  the  surface,  as  well  as  the  color  of  the 
section.  In  transmitted  light  we  have  a  new  color  classification, 
wherein  some  fillers  which  are  opaque  and  colored  in  reflected 
light  may  be  translucent  and  show  a  different  color  by  trans- 
mitted light.  In  polarized  light,  we  have  the  differences  in  opti- 
cal behavior  between  crystalline  and  non-crystalline  substances; 
interference  figures,  extinction  angles,  etc.,  to  further  classify  the 
materials  under  observation.  Considering  the  comparatively 
limited  number  of  substances  one  finds  in  rubber  compounds,  as 
compared  with  the  entire  mineral  field,  the  possibility  of  exact 
identification  is  very  great. 

As  far  as  the  identification  of  fillers  is  concerned,  the  future 
seems  bright,  and  today  practically  all  the  work  is  being  con- 
ducted along  these  lines.  We  have  still  to  consider  the  possibility 
of  identifying  different  rubbers,  or  the  rubber  plastics,  such  as 
the  mineral  rubbers,  substitutes,  etc.,  reclaimed  rubber,  soften- 
ing oils  and  waxes,  etc.  For  some  of  these  substances,  notably 
mineral  rubber,  paraffin,  rosin,  oil  substitutes,  we  have  excellent 
chemical  means  of  identification,  and  more  or  less  accurate  means 
for  their  quantitative  determination.  The  problem  of  the  iden- 
tification of  reclaimed  rubber,  and  the  different  grades  of  new 
rubber,  is  still  open  for  solution,  and  it  may  be  that  this  new 
means  of  research  will  prove  of  valuable  assistance  in  investiga- 
tions of  this  sort. 

8  Some  excellent  text  books  for  this  type  of  work  are  found  in  "Minerals  in 
Rock  Sections,"  by  Luquer,  D.  Van  Nostrand  Co.,  and  "Characters  of  Crystals," 
by  A.  J.  Moses,  D.  Van  Nostrand  Co.  The  preparation  and  identification  of 
minerals  in  rock  sections,  measurement  of  crystal  faces,  extinction  angles,  lines 
of  cleavage,  etc.,  will  be  excellent,  and  withal  comparatively  simple  preparation 
for  the  study  of  microsections  of  rubber.  Fred  E.  Wright  (see  bibliography) 
has  done  some  excellent  work  in  the  field  of  the  identification  of  minerals  in 
rocks,  through  the  aid  of  the  petrographic  microscope,  and  any  one  attempting 
work  in  the  field  of  the  microscopic  examination  of  rubber  compounds  will  find 
a  careful  study  of  Wright's  work  to  be  of  great  help. 


Chapter  XII. 
Calculation  to  Approximate  Formulas- 

The  greater  number  of  analyses  are  made  for  the  purposes  of 
checking  factory  production,  and  for  comparing  finished  goods 
sold  under  chemical  specifications.  In  such  cases,  a  complete 
analysis  is  seldom  desired;  for  factory  purposes,  a  few  deter- 
minations suffice,  and  for  specification  purposes  the  analysis  is 
carried  just  far  enough  to  decide  whether  or  not  the  specifications 
have  been  complied  with.  In  the  latter  case,  it  is  usually  suf- 
ficient to  report  the  percentage  of  the  rubber  present,  the  pres- 
ence or  absence  of  reclaimed  rubber,  the  free,  total,  and  barium 
sulfate-sulfur,  the  presence  and  approximate  amounts  of  oils, 
waxes,  mineral  rubbers,  substitutes,  and  any  other  organic  fillers 
likely  to  have  a  bearing  on  the  analysis. 

There  are  times  when  one  is  interested  in  learning  everything 
concerning  an  article,  and  then,  in  addition  to  the  foregoing,  we 
need  a  complete  analysis  of  the  mineral  fillers,  both  as  to  the 
basic  and  acidic  radicles.     From  these  data,  we  build  up  an 
approximate  formula.    The  report  of  the  analysis  should  cover 
the  following  points: 
Rubber  hydrocarbons 
Acetone  extract,  sulfur  free 

Color  and  appearance  of  extract 
Saponifiable  matter 
Unsaponifiable  matter 
Mineral  hydrocarbons 
Vegetable  hydrocarbons 
Chloroform  extract 

Color  and  appearance  of  extract 
Alcoholic  potash  extract 

Color  and  appearance  of  extract 
Total  sulfur 
Free  sulfur 
Sulfur  of  Vulcanization 

117 


118  THE  AKALYMt  OF  RUBBER 

Glue 
Carbon 

Other  organic  fillers 
Mineral  Fillers 
Bases 

Aluminium  as  Al,0, 
Antimony  as  Sb2Sa 
Barium  as  BaO 
Calcium  as  CaO 
Iron  as  Fe203 
Lead  as  PbO 
Magnesium  as  MgO 
Zinc  as  ZnO 
Any  other  bases 
Acids 

Carbonate  as  C02 
Silica  as  Si02 
Sulfide-sulfur  as  S 
Sulfite-sulfur  as  S02 
Sulfate-sulfur  as  S03 
Organic  Accelerators 
Specific  Gravity 

With  these  data  before  us,  we  may  proceed  with  the  recon- 
struction of  the  compound. 

Rubber.  The  rubber  is  the  sum  of  the  rubber  hydrocarbons 
(sulfur  free),  and  the  acetone,  chloroform  and  alcoholic  potash 
extracts,  providing  that  no  organic  matter,  other  than  that 
originally  present  in  the  rubber,  is  shown  by  the  analyses. 
Ordinarily,  with  new  rubber,  the  acetone  extract  will  not  exceed 
4%,  the  chloroform  extract  in  a  properly  cured  article  2%,  and 
the  alcoholic  potash  extract  1%.  based  upon  the  rubber.  If  any 
appreciable  quantity  in  excess  of  these  amounts  is  found,  it  must 
be  explained. 

Sulfur.  The  sulfur  added  as  such  is  the  sum  of  the  free  sulfur 
and  the  sulfur  of  vulcanization,  plus  any  sulfur  which  may  have 
combined  with  the  fillers  during  vulcanization.  This  latter  item 
is  often  difficult,  and  sometimes  impossible  to  determine,  but  a 
knowledge  of  the  general  procedure  in  designing  rubber  com- 
pounds will  be  a  help. 

Organic  Fillers.     The   oils,   fats,  waxes,  etc.,  are   determined 


CALCULATION  TO  APPROXIMATE  FORMULAS  119 

from  tests  on  the  acetone,  chloroform  and  alcoholic  potash  ex- 
tracts. Mineral  rubber  at  best  can  be  only  approximated. 
Special  fillers,  such  as  glue,  cellulose,  carbon,  etc.,  are  set  down 
just  as  they  are  determined. 

Inorganic  Fillers.  With  a  knowledge  of  what  bases  and  acids 
are  present,  we  may  start  to  build  up  the  composition  of  the 
mineral  fillers. 

Antimony  Compounds.  If  only  antimony,  sulfur  and  calcium 
sulfate  are  found,  in  addition  to  the  rubber,  we  know  that  we 
have  a  mixture  of  golden  sulfide  and  rubber,  and  not  only  is 
the  calculation  simple,  but  also  we  have  the  formula  of  the 
golden  sulfide  used. 

Barium.  All  barium  should  be  calculated  to  sulfate,  unless 
by  analysis  barium  carbonate  is  shown  to  be  present. 

Calcium.  In  the  absence  of  antimony,  calcium  may  be  calcu- 
lated to  the  carbonate,  unless  the  quantity  present  is  less  than 
\%.  In  such  cases,  especially  in  the  absence  of  reclaimed  rubber, 
it  may  be  assumed,  with  some  assurance,  that  this  small  amount 
was  added  as  hydrated  lime.  In  the  presence  of  whiting,  the, 
hydrated  lime  cannot  be  detected. 

Aluminium.  Aluminium  is  probably  present  as  a  silicate.  The 
microscope  will  be.  found  to  be  an  absolute  necessity  to  determine 
which  silicate  is  present.  In  the  absence  of  magnesium  a  white 
compound  will  probably  contain  aluminum  flake,  or  white  clay. 
Some  clays  contain  titanium,  and  a  positive  qualitative  test  for 
titanium  would  be  sufficient  indication  that  the  substance  is 
clay.  Titanium  oxide,  associated  with  barium  sulfate,  is  used 
as  a  paint  pigment,  but  only  in  an  experimental  way  in  rubber. 

Iron.  Iron  is  usually  present  as  the  oxide,  but  frequently  is 
associated  with  clay.  It  is  sufficient  for  the  purpose  to  report 
the  oxide  and  clay  separately;  then,  in  rebuilding  the  compound, 
any  clay  in  excess  of  that  found  in  the  iron  oxide  used  must  be 
added  as  such. 

Lead.  Without  question,  lead  is  one  of  the  most  difficult  sub- 
stances to  work  upon.  If  organic  accelerators  are  present,  it  is 
probable  that  lead  oleate,  sublimed  white  or  blue  lead  is  present. 
Probably  as  safe  a  thing  to  do  as  any  is  to  work  up  all  of  the 
other  fillers  first,  and  then  apply  any  sulfide,  sulfite,  or  sulfate- 
sulfur  to  the  lead. 

Magnesia.    Magnesia  may  be  present  in  one  of  three  forms,  the 


120  THE  ANALYSIS  OF  RUBBER 

oxide,  carbonate,  or  silicate.  With  silica  present,  and  no 
aluminium,  a  magnesium  silicate  is  probable.  In  the  absence  of 
whiting,  any  carbon  dioxide  found  is  probably  combined  with 
magnesium,  although  lead  carbonate  (white  lead)  may  interfere. 
The  specific  gravity  of  the  compound  as  a  whole  is  one  means 
for  distinguishing  between  the  oxide  and  carbonate. 

Zinc.  Zinc  is  usually  present  as  the  oxide,  and  the  simul- 
taneous presence  of  barytes  is  not  evidence  that  lithopone  is 
present.  In  the  absence  of  lead  and  antimony,  any  sulfide-sulfur 
will  undoubtedly  be  combined  with  zinc.  It  is  best  to  calculate 
all  zinc  as  the  oxide,  and  not  to  assume  that  lithopone  is  present 
unless  there  is  an  excess  of  sulfide-sulfur  over  that  required  for 
lead  or  antimony. 

After  the  approximate  amount  of  the  probable  ingredients  of 
the  compound  have  been  worked  out  as  above,  the  sum  should  be 
in  the  neighborhood  of  100% — if  anything,  should  exceed  that. 
The  next  step  is  to  take  this  formula  and  calculate  the  specific 
gravity,  which  should  check  within  0.02  the  specific  gravity  of 
the  original  compound.  Any  greater  discrepancy  than  this  re- 
veals some  error,  which  must  be  checked  up.  Obviously,  if  our 
calculations  are  low,  the  high  gravity  substances  are  in  error, 
and  vice  versa.  If  the  gravities  agree  closely,  then  the  figures 
may  be  rounded  off  to  even  percentages,  to  the  nearest  0.25%, 
and  brought  by  adjustment  exactly  to  100%. 

It  must  be  very  clear  to  every  one  that  the  interpretation  of 
analytical  results  is  a  matter  requiring  experience,  ingenuity,  and 
a  great  deal  of  common  sense.  The  intent  of  the  above  is  cer- 
tainly not  to  lay  down  exact  rules,  but  merely  to  indicate  the 
general  line  of  thought,  permitting  the  analyst,  with  his  first-hand 
information  as  to  the  progress  of  the  analysis,  to  make  such 
deductions  as  may  seem  wise. 


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121 


122 


THE  ANALYSIS  OF  RUBBER 


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Gummi  Ztg.  27,  2087. 
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39.  Determination  of  mineral  rub- 

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40.  Discussion  of  Hubener's  tetra- 

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42.  Reactions  of  accelerators  dur- 

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43.  Reactions  of  accelerators  dur- 

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44.  Analysis  of  manufactured  soft 

and     hard     rubbers.      Ann. 
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45.  A  method  for  the  direct  deter- 

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46.  Contribution   to  the  study  of 

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51.  Vulcanization     of    rubber    by 

selenium.      J.      Ind.      Eng. 
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L.  M .  Bourne 

52.  Resin  and  sulfur  in  India  rub- 

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Jean  Boutaric 

53.  Analysis  of  rubberized  fabrics. 

Caoutchouc    &    Guttapercha 
17,  10202-6. 
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54.  Use     of    pyridine     in    rubber 

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55.  Ceresin  wax  in  rubber  mixings. 

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56.  Rapid  determination  of  fillers 

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123 


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58.  The  presence  of  manganese  in 

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59.  The   tackiness    of  raw   rubber 

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60.  Mechanism   of   action   of  cer- 

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124 


THE  ANALYSIS  OF  RUBBER 


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103.  Relation  between  specific  grav- 

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108.  The  influence  of  zinc  oxide  on 

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109.  The    melting   points    of   some 

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670  (1907). 

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111.  The  dyeing  of  rubber  with  or- 

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1162. 


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125 


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of  rubber.  Gummi  Ztg. 

113.  Effect    of    powdered    glass    on 

21,  234-5  (1907). 
Rudolf  Ditmar  and  Thieben 

114.  Changes  occurring  in  the  most 

important     inorganic     fillers 
during    steam   vulcanization. 
Koll.  Z.  11,  77-80. 
E.  D.  Donaldson 

115.  Rapid  electrolytic  method  for 

total  lead  and  zinc  in  rubber 
compounds.     Chem.  Analyst 
15,  11-12  (1915);  India  Rub- 
ber J.  57,  1100  (1919). 
Andre  Dubosc 

116.  Action  of  ozone  and  oxozone 

in  the  analysis  of  rubber. 
Caoutchouc  &  Guttapercha 
10,  7105  (1913). 

117.  Analysis  of  vulcanized  rubber. 

Caoutchouc  &  Guttapercha 
13,  8782-3  (1916). 

118.  Method  of  determination  and 

identification  of  proteins  in 
rubber. '  Caoutchouc  &  Gut- 
tapercha 13,  8810-1  (1916). 

119.  The    analysis    of    rubber    ma- 

terials. Caoutchouc  & 
Guttapercha  13;  8939-44; 
8980-3  (1916). 

120.  The    role    of    analysis    in    the 

manufacture  of  rubber 
goods.  Caoutchouc  &  Gut- 
tapercha 13,  9055  (1916). 

121.  Action  of  amines  in  vulcaniza- 

tion. Caoutchouc  &  Gutta- 
percha, 13,  9064  (1916). 

122.  Changes    in    resin    content    of 

rubber  on  vulcanization. 
Caoutchouc  <fe  Guttapercha 

13,  9094-5  (1916). 

123.  Physical  and  chemical  analysis 

of  rubber  thread.  Caout- 
chouc &  Guttapercha  13, 
9007-8  (1916). 

124.  Analysis  of  vulcanized  rubber. 

Caoutchouc  &   Guttapercha, 

14,  9189-91     (1917);    9213-6 
(1917);  9309-13  (1917). 

125.  Application     of     catalysis     to 

vulcanization.  Rubber  Age 
3,  78-9  (1918). 

126.  A    comparison    of    the    effects 

produced  by  organic  accel- 
erators in  the  vulcanization 


128. 

129. 
130. 
131. 
132. 

133. 

134. 

135. 
136. 
137. 
138. 
139. 

140. 

141. 
142. 


of  rcreDer.  Caoutchouc  & 
Guttapercha  15,  9635-7 
(1918). 

bber  substitutes,  or  vulcan- 
ized  oils.  Chimie  &  Indus- 
trie 1,  727-32  (1918). 

Polymerization  and  oxidation 
of  crude  rubber.  Caoutchouc 
&  Guttapercha  15,  9478-82 
(1918). 

Action  of  lipase  on  white  fac- 
tice.  Caoutchouc  &  Gutta- 
percha, 16,  9722-7  (1919). 

Vulcanization  accelerators. 
Caoutchouc  &  Guttapercha 
16,  9853;  9856-65  (1919). 

Analysis  of  vulcanized  rubber. 
Caoutchouc  &  Guttapercha 
16,  9900-7  (1919). 

Determination  of  sulfur  exist- 
ing as  sulfides  in  vulcanized 
rubber.  Caoutchouc  &  Gut- 
tapercha 16,  9952-3  (1919). 

The  use  of  furfural  in  rubber 
analysis  and  in  the  rubber 
industry.  Caoutchouc  & 
Guttapercha  16,  9957-9 
(1919). 

Estimation  of  rubber  and  tex- 
tiles in  impermeable  fabrics. 
Caoutchouc  &  Guttapercha 
16,  9907-11  (1919). 

Analysis  of  vulcanized  rubber. 
Caoutchouc  &  Guttapercha 
16,  9901-6  (1919). 

Analysis  of  crude  rubber. 
Caoutchouc  &  Guttapercha 

16,  10051-3  (1919). 

The  use  of  carbon  black  in 
tires.  Caoutchouc  &  Gutta- 
percha 17,  10274-5  (1920). 

The  discovery  of  accelerators. 
Caoutchouc  &  Guttapercha 

17,  10427  (1920). 

Theory  of  the  acceleration  of 
vulcanization.  Caoutchouc 
&  Guttapercha  17,  10511-4 
(1920). 

Application  of  some  amides 
and  amines  of  furfural  to 
vulcanization.  Caoutchouc 
&  Guttapercha  17,  10495-505 
(1920). 

The  new  vulcanization  and 
accelerators.  I.  Caoutchouc 
&  Guttapercha  18,  11012-5 
(1921). 

Ibid.  II.  Caoutchouc  &  Gut- 
tapercha 18,  11121-4  (1921), 


126 


THE  ANALYSIS  OF  RUBBER 


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144.  The  new  vulcanization  and  ac- 

celerators.      Caoutchouc       it 
Guttapercha        If).        11171-6 
(1922). 
Andre   Dubose  and  J<an   Wart  It  I 

145.  Mineral    Rubber.     Caoutchouc 

&  Guttapercha  in.  10037-40 
(1919). 

146.  Action    of   Upases   on    oils   vul- 

canized   with    sulfur   chloride 
(white       substitutes).       Bu.ll. 
Soc.    Ind.    Ron  on    .;?'.    47-59* 
(1919). 
H'.  A.  Dneca 

147.  The    determination    of    rubber 

as    a    tetrabromide.      .1.    Ind. 
Kng.  Chem.  4,  372-4    (1912). 
Richard  B.  Earh- 

148.  Report    of    Committee    on    or- 

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149.  Vulcanization    catalysts.      Agr. 

Bull.  Federated  Malay  States 
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150.  The     natural     accelerators     ot 

Para    rubber.     J.  Soc.   Chem. 
Ind.  37,  51T   (1918). 
H.  ./.  Eaton  and  F.  W .  Den/ 

151.  The    distribution     of    nitrogen 

in    coagulum    and    serum    of 
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350-3  (1916). 

152.  A  preliminary  investigation  on 

the  estimation  of  free  and 
combined  sulfur  in  vulcan- 
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of  combination  of  sulfur  with 
different  types  of  plantation 
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153.  Investigation   on  sulfur   in   vul- 

canized   rubber.      Agr.    Bull. 
Federated     Malay     States     I',, 
73-87    (1917). 
Jiinius  David  Edirards 

154.  Methods  of  exposure   and   per- 

meability tests  of  balloon 
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for  Aeronautic-,  459-63 
(1917). 


N. 
156. 


X.  I 
158. 


.  End  res 

The    relative    activity    of    cer- 

tain accelerators  In  the  vul- 

canization of  rubber.    Caout- 

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11089-97  (1921). 
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The      extraction      of      rubber 

goods.     Rubber   Age   6,   445- 

7  (1920). 

.  Epxtcin  and  W.   E.  Lanae 
Detection     and     determination 

of     glue     in     rubber     goods. 

India  Rubber  World  fil  .  216- 

7   (1920). 

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rubber  goods.     V.  S.  Bureau 

of     Standards     Tech.     Paper 

154;    Rubber    Ago   6.    289-93 

(1920). 


159.  Determination     of     sulfur     in 

rubber.  Chem.  Ztg.  28,  200 
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160.  Contributions     to     rubber     in- 

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22.  766. 

161.  Application     of     the     bromide 

derivative    methods,    for  the 
determination    of  vulcanized 
rubber    goods.      Chem.    Ztg. 
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IF.  W.  E  raws  and  JRuth  McTling 

162.  A  rapid  bomb  method  for  de- 

termination of  sulfur  in  rub- 
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1921. 
Ci.  Fendler 

163.  New    method    for    the   analysis 

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164.  Determination     of     rubber     as 

tetrabromide.  Gummi  Ztg. 
,?,;.  782  (1910). 

165.  Determination     of     rubber     as 

nitrosite.      Gummi    Ztg.    ,'?,/, 
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166.  Studies   on    rubber  and    rubber 

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167.  The    determination    of    rubber 

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Ztg.  9,2,  710  (1908), 


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258.  Suggestions    for    the    standard 

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260.  A    new    extraction    apparatus. 

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261.  See  W.  A.  Del  Mar 
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262.  Analysis  of  rubber  compounds. 

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264.  Methods  of  determining  small 

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275.  The     regeneration    of    rubber 

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276.  The    action    of    concentrated 

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265.   A  simple  method  for  the  de-      27&   Syntheses   of  rubber.     Caout- 


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281.  Estimation  of  mineral  matter 

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268.   The  determination  of  the  true      282.   Critical    investigation    of    the 
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134 


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tetrabromide ;  decomposition 
of  the  bromide  by  nitric 
acid.  Gummi  Ztg.  25,  301-3 
(1911).  Cf.  also  Gummi 
Markt.  5,  113. 
D.  Spence  and  G.  D.  Kratz 

393.  The    insoluble    constituent    of 

crude    rubber;    its    isolation 
and    characterization.     Koll. 
Z.  14,  262  (1914). 
D.  Spence  and  C.  A.  Ward 

394.  The  Chemistry  of  India  rub- 

ber. IV.  The  theory  of 
vulcanization.  Z.  Chem. 
Ind.  Kolloide  11,  274 
(1913). 

D.  Spence  and  J.  Young 

395.  Comparison  of  some  methods 

for  the  estimation  of  sulfur 
in  vulcanized  rubber,  with 
especial  reference  to  electro- 
lytic oxidation.  J.  Ind.  Eng 
Chem.  4,  413  (1912). 

E.  Stern 

396.  Technical  reports  of  the  Sec- 

ond    International     Rubber 
Conference     in     London. 
Gummi  Ztg.  25,   1570;   1604 
(1911). 
H.  P.  Stevens 

397.  The  influence  of  various  nitro- 

gen and  resinous  substances 
on  the  vulcanizing  properties 
of  rubber.  India  Rubber  J. 
47,  403-5. 

398.  The    estimation    of    sulfur    in 

rubber.  Analyst  39,  74; 
India  Rubber  J.  47,  785. 

399.  The  estimation  of  sulfide  and 

sulfate-sulfur  and  the  action 
of  solvents  on  vulcanized 
rubber.  Analyst  40,  275- 
81. 

400.  The    vulcanization    of    rubber 

by  agents  other  than  sulfur. 
J.  Soc.  Chem.  Ind.  35,  107-9 
(1917). 

401.  The    coefficient    of    vulcaniza- 

tion and  the  state  of  cure. 
India  Rubber  J.  53,  220-3 
(1917). 

402.  The  function  of  litharge  in  the 

vulcanization  of  rubber;  the 
influence  of  the  resinous  con- 
stituents. J.  Soc.  Chem.  Ind. 
35,  874-7  (1916). 


403.  The     natural     accelerator     of 

Para  rubber.  J.  Soc.  Chem. 
Ind.  36,  365-70  (1917). 

404.  Comparative  methods  for  de- 

termining the  state  of  cure 
of  rubber.  J.  Soc.  Chem. 
Ind.  37,  280-4T  (1918). 

405.  The    nature    of   vulcanization. 

J.  Soc.  Chem.  Ind.  39. 192-6T 
(1919). 

406.  The     comparative     effect     of 

organic  and  inorganic  accel- 
erators in  vulcanizing  rub- 
ber. J.  Soc.  Chem.  Ind.  37, 
156-8T  (1918). 

407.  The  determination  of  sulfur  in 

vulcanized  rubber.  Analyst 
48,  377-8  (1918). 

408.  The  comparative  effect  of  or- 

ganic accelerators  and  mag- 
nesia in  vulcanizing  rubber. 
India  Rubber  J.  58,  527-31 
(1919). 

409.  Notes  on  Accelerators.     India 

Rubber  World  62,  719-20 
(1920). 

410.  Methods     of     testing.     Bull. 

Rubber  Growers  Assocn.  1, 
34-8  (1919). 

411.  The  analysis  of  rubber.  Caout- 

chouc &  Guttapercha  17, 
10455-8  (1920). 

412.  The  aging  of  vulcanized  rub- 

ber.    Bull.  Rubber  Growers 
Assocn.  2,  270-6  (1920). 
Thale 

413.  The      analysis      of      ebonite. 

Chem.  Ztg.  30,  499  (1906). 
M.  Toch 

414.  The  influence  of  pigments  on 

rubber.    J.  Ind.  Eng.  Chem. 
9,  694-6  (1917). 
J.  Torrey 

415.  A  new  method  for  determin- 

ing rubber.    India  Rubber  J. 
30,  417-8  (1905). 
Tschirch  and  Schmitz 

416.  Method    for    determining    ni- 

trogenous      substances       in 
crude  rubber.     Gummi  Ztg. 
26,  2079  (1912). 
John  B.  Tuttle 

417.  The  sampling  of  rubber  goods. 

J.  Ind.  Eng.  Chem.  5,  618 
(1913).- 

418.  The  determination  of  barium 

carbonate  and  barium  sul- 
fate  in  rubber  goods.  U.  S. 
Bureau  of  Standards  Tech. 


136 


THE  ANALYSIS  OF  RUBBER 


Paper  64;  J.  Ind.  Eng.  Chem. 
8,  324-6  (1916). 

419.  Chemical  tests  of  balloon  fab- 

rics. Third  Annual  report, 
Nat.  Advisory  Comm.  for 
Aeronautics  1917,  463-6. 

420.  Action    of  heat   and   light   on 

vulcanized  rubber.  The 
Rubber  Age  8,  271-2 
(1921). 

421.  Guides   to   analyses;    The   in- 

terpretation of  rubber  analy- 
sis. Chem.  Bull.  7,  332-4 
(1920). 

422.  The  variability  of  crude  rub- 

ber.   J.  Ind.  Eng.  Chem.  13, 
519-22    (1921). 
John  B.  Tuttle  and  A.  Isaacs 

423.  A     study     of     some     recent 

methods  for  the  determina- 
tion of  total  sulfur  in  rub- 
ber. U.  S.  Bureau  of 
Standards  Tech.  Paper  45; 
J.  Ind.  Eng.  Chem.  7,  658- 
63  (1915). 
John  B.  Tuttle  and  Louis  Yurow 

424.  Preliminary  note  on  the  direct 

determination  of  rubber. 
India  Rubber  World  67,  17- 
8  (1917). 

425.  The    direct    determination    of 

India  rubber  by  the  nitrosite 
method.     U.    S.   Bureau    of 
Standards   Tech.   Paper    145 
(1919) ;  C.  A.  14,  1063. 
Douglas  F.  Twiss 

426.  Hydrochloric    acid-ether    mix- 

ture as  a  reagent  for  rubber 
analysis.  India  Rubber  J. 
60,  199  (1915). 

427.  C.   O.   Weber's   chemical   test 

for  sun  cracking.  India 
Rubber  J.  52,  325  (1916). 

428.  The    chemistry    of    vulcaniza- 

tion. J.  Soc.  Chem.  Ind.  36, 
782-9  (1917). 

429.  Vulcanization       catalysts.      J. 

Soc.  Chem.  Ind.  36,  1072 
(1917). 

430.  The   discontinuity   of   vulcan- 

ization in  the  presence  of 
organic  accelerators.  J.  Soc. 
Chem.  Ind.  40,  242-8T 
(1921). 

431.  Determination      of      available 

sulfur  in  golden  sulfide  of 
antimony.  J.  Soc.  Chem. 
Ind.  41,  20T  (1922). 

432.  Rubber  as  a  colloid.      Caout- 


chouc    &     Guttapercha    17, 
10240-3  (1920). 
7).  7^.  Twiss  and  S.  A.  Brazier 

433.  Acceleration   of   vulcanization. 

J.  Soc.  Chem.  Ind.  39, 
125-32T;  155-6T;  287T 
(1920). 

D.  F.  Twiss,  S.  A.  Brazier,  and  F. 
Thomas 

434.  The      dithiocarbamate      accel- 

erators of  vulcanization.    J. 
Soc.    Chem.   Ind.  41,  81-8T 
(1922). 
D.  F.  Ttwss  and  G.  Martin 

435.  The  detection  of  accelerators. 

The   Rubber   Age   9,   379-80 
(1921). 
S.  Uchida 

436.  Notes  on  some  fatty  oils  (in- 

cluding Para  rubber  tree 
oil).  J.  Soc.  Chem.  Ind.  35, 
1089-90  (1916). 

A.  J.  Ultee 

437.  Stearins    from    Castilloa    and 

Ficus  rubber.  Chem.  Zentr. 
II,  1469  (1912). 

438.  Resin  content  of  Ficus  rubber. 

Gummi  Ztg.  26,  1427  (1912). 

B.  Vnger 

439.  Analysis  of  vulcanized  rubber. 

and    the    estimation   of    the 
sulfur  in  the  same.    Z.  anal, 
chem.  24,  167-75. 
H.  S.  Upton 

440.  Volumetric    determination    of 

free    sulfur    in    soft    rubber 
compounds.      J.     Ind.     Eng. 
Chem.  10,  518-20   (1918). 
Utz 

441.  Determination    of    rubber    as 

tetrabromide.  Gummi  Ztg. 
26,  968  (1912). 

442.  Estimation    of    mineral    fillers 

in  rubber  goods.  Kunst- 
stoffe  2,  381;  409. 

443.  The    investigation    of    golden 

antimony.  Gummi  Ztg.  28, 
126  (1914). 

444.  The  determination  of  total  sul- 

fur in  rubber.  Gummi  Ztg. 
28,  631-2  (1914). 

445.  Recent  methods  for  the  esti- 

mation of  rubber.    Z.  angew. 
Chem.  S2,  235-6  (1919). 
Vallery 

446.  Application  of     Koettstorfer's 

Index  to  the  determination 
of  resins  and  adulterations 
in  rubber.  Mon.  sci.  8,  82. 


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137 


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448.  The      properties      of     factice. 

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452.  Determination  of  the  particle 

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457.  Influence  of  factors  in  rubber 

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458.  A   new   modification   of   Hen- 

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Jean  Wavelet 

462.  Observations   on  the  work   of 

Stevens,  and  of  Kratz  and 
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463.  On  the  analysis  of  India  rub- 

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Ind.  13,  476-85  (1894). 

464.  Concerning    the    vulcanization 

of  rubber.  Chem.  Ztg.  18, 
837-8;  1695-6  (1894). 

465.  Upon   the   analysis   of  rubber 

goods.  Chem.  Ztg.  18, 
1003-5;  1040-1;  1064-9 
(1894). 

466.  On  the  vulcanization  of  India 

rubber.  J.  Soc.  Chem.  Ind. 
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467.  The     analysis     of     vulcanized 

rubber.  Z.  angew.  chem.  11, 
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468.  The    nature   of   India   rubber. 

J.  Soc.  Chem.  Ind.  19,  215- 
21  (1900). 

469.  The  determination  of  sulfur  in 

rubber.  Gummi  Ztg.  17, 
179-80  (1902). 

470.  Analysis      of      rubber     goods. 

Gummi  Ztg.  17,  207-8  (1902). 
Cf.  also  Ber.  36,  3103;  3108 
(1903). 

471.  Methods  for  the  determination 

of  rubber.  Gummi  Ztg.  18, 
461  (1903). 

472.  A    new     method     for    rubber 

analysis.  Gummi  Ztg.  18, 
339-41;  521-3  (1903). 

473.  The  use  of  chloral  hydrate  in 

rubber  analysis.  India  Rub- 
ber J.  25,  375  (1903). 

474.  The  action  of  light  on  India 

rubber.  India  Rubber  J.  26, 
639  (1903). 

475.  The   influence    of   litharge    on 

hot  vulcanization.  Gummi 
Ztg.  17,  296  (1903). 


138 


THE  ANALYSIS  OF  RUBBER 


Lothar  E.  Weber 

476.  Preparation  of  rubber  mixings 

for  analysis.  Gummi  Ztg. 
21,  797  (1907). 

477.  The  action  of  resins  in  the  vul- 

canizing of  rubber.  8th  Int. 
Cong.  Appl.  Chem.  27, 
71-2. 

478.  Increase  of  resins  in  the  vul- 

canization. India  Rubber 
World  55,  6  (1916). 

479.  The  nature  and  uses  of  rub- 

ber solvents.  India  Rubber 
World  66,  317-8;  377-8 
(1917). 

480.  The  use  of  magnesia  in  rubber 

compounds.      India    Rubber 
World  57,  209   (1918). 
An    examination    of    German 
'synthetic       rubber.       India 
Rubber     World      61,     71-2 
(1919). 
Weber  and  Sweet 

482.  Antimony  sulfide.    Caoutchouc 

&      Guttapercha      15,     9468 
(1918). 
L.  G.  Wesson 

483.  Preliminary    note    on    a    new 

method  for  the  direct  deter- 
mination of  rubber.  J.  Ind. 
Eng.  Chem.  5,  398  (1913). 

484.  The    combustion    method    for 

the  direct   determination   of 
rubber.    J.  Ind.  Eng.  Chem. 
6,  459-62  (1914). 
L.  G.  Wesson  and  E.  S.  Knorr 

485.  Wet  combustion  in  the  nitro- 

site  combustion  method  for 
the  direct  determination  of 
rubber.  India  Rubber  World 
55,  69  (1916).  J.  Ind.  Eng. 
Chem.  9,  139-40  (1917). 
R.  Wheatly  and  B.  D.  Porritt 

486.  A  new  machine  for  the  prep- 

aration of  vulcanized  rubber 
for  analysis.     J.  Soc.  Chem. 
Ind.  34,  587-8  (1915). 
G.  Stafford  Whitby 

487.  Moisture  in  crude  rubber.     J. 

Soc.  Chem.  Ind.  37,  278-80T 
(1918). 

488.  Variation  in  Hevea  Brazilien- 

sis.  Ann.  Bot.  31,  312-21 
(1919). 

489.  The       caoutchouc      molecule. 

India  Rubber  J.  62,  313 
(1921). 


G.  Stafford  Whitby  and  J.  Doolid 

490.  Contributions    to    the    knowl- 

edge of  the  resins  of  Hevea 
rubber.     Rochester    meeting 
of   the    American    Chemical 
Society,  April,  1921. 
G.    Stafford     Whitby    and    0.    J. 
Walker 

491.  Influence    of    certain    organic 

accelerators  on  the  vulcan- 
ization of  rubber.  J.  Ind. 
Eng.  Chem.  13,  816-9  (1921). 

492.  The    influence    of    piperidine- 

piperidyl  dithiocarbamate  on 
vulcanization.  Rubber  Age 
9,  455-60  (1921). 

G.  Stafford  Whitby  and  Arnold  H. 
Smith 

493.  Rapid    and    low    temperature 

vulcanization.      New      York 
meeting    of    the     American 
Chemical    Society,    Septem- 
ber, 1921. 
Ernst  Wleck 

494.  Contribution  to  the  investiga- 

tion of  golden  sulfide;  detec- 
tion   and    determination    of 
calcium  sulfate.  Gummi  Ztg. 
29,  479-80  (1915). 
Van  Lear  Woodward 

495.  Antimony  sulfide;  its  place  in 

tube   making.     Rubber   Age 
1,  99-100  (1917). 
Fred  E.  Wright 

496.  Oblique  illumination  in  petro- 

graphic  microscope  work. 
Am.  J.  Sci.  35,  63-82. 

497.  The   measurement   of  the   re- 

fractive index  of  a  drop  of 
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498.  Determination  of  the  relative 

refringence  of  mineral  grains 
under  the  petrographic  mi- 
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389-92;  5,  101-7. 

499.  Recent    improvements    in   the 

petrographic  microscope.  J. 
Wash.  Acad.  6,  465-7  (1916) ; 
C.  A.  10,  2419. 

500.  Petrographic      microscope      in 

analysis.  J.  Am.  Chem.  Soc. 
38,  1647-58  (1916). 

501.  Polarized  light  in  the  study  of 

ores  and  metals.  Proc.  Am. 
Phil.  Soc.  58,  401-47  (1919) ; 
C.  A.  14,  1645. 


APPENDIX  A. 

THE  PREPARATION  OF  MATERIALS  FOR  RUBBER  MANUFACTURE. 

While  to  many  readers,  the  factory  processes  concerned  with  the  prep- 
aration of  the  materials  which  enter  into  the  composition  of  a  rubber 
compound  are  every  day  affairs,  for  the  benefit  of  those  who  are  not 
familiar  with  the  technical  processes  used  in  rubber  factories,  it  seems 
worth  while  to  give  some  brief  description  of  the  various  steps.  The  crude 
rubber  is  prepared  by  washing,  if  necessary,  and  by  ''milling"  or  "breaking 
down";  the  pigments  may  be  screened  or  bolted.  Certain  pigments  and 
organic  accelerators  are  incorporated  with  rubber  to  form  master  batches. 
These,  together  with  such  pigments  and  crude  rubbers  as  are  used  in  the 
form  in  which  they  are  received,  are  fed  to  the  compound  room,  where 
the  batches  are  weighed  out  according  to  the  prescribed  formulas.  These 
batches  are  mixed  in  the  mill  room,  and  then  stored  to  cool  and  age. 
The  mixed  rubber  compound  is  prepared  for  manufacturing  purposes  by 
(a)  calendaring  into  sheets  or  strips;  (b)  tubing  into  special  or  irregular 
shapes;  (c)  made  into  a  rubber  cement  by  dissolving  the  rubber  com- 
pound in  benzene,  gasoline,  or  a  mixture  of  the  two;  (d)  applied  to  fab- 
ric, either  on  a  calendar,  or- on  a  spreading  machine.  These  four  classes 
may  be  called  the  "intermediates"  of  the  rubber  industry;  from  these, 
practically  all  rubber  articles  are  built. 

Washing.  The  wild  rubbers,  and  the  inferior  grades  of  plantation  rubber, 
contain  large  amounts  of  bark,  dirt,  and  other  foreign  matter,  and  in  this 
condition  are  not  suitable  for  rubber  manufacture.  These  impurities  are 
removed  by  washing.  The  better  grades  of  plantation  rubber  may  be 
washed  in  order  to  be  certain  that  no  grit  or  dirt  remains  in  the  rubber, 
to  cause  trouble  during  the  service  of  the  articles  of  which  they  may  form 
a  part. 

The  washing  equipment  consists  of  a  vat  for  heating  the  rubber,  and 
three  _  washing  mills — a  cracker,  a  refiner,  and  a  finisher.  Some  factories 
combine  the  refining  and  finishing  on  one  mill. 

Some  of  the  wild  rubbers,  notably  the  fine  Para  sorts,  come  in  large 
lumps,  or  biscuits.  Before  they  are  ready  for  washing,  they  must  be  boiled 
in  water  for  several  hours  in  order  to  soften  them  up  sufficiently  to  be 
worked  easily.  The  large  lumps  are  cut  up  before  being  heated. 

The  cracker  consists  of  two  rolls  with  heavy,  coarse  corrugations.  The 
rubber  is  passed  through  this  mill  two  or  three  times,  just  enough  to  form 

thick,  rough  sheet. 

The  refiner  is  a  two-roll  mill,  the  front  roll  having  finer  corrugations 
than  the  cracker.  A  stream  of  water  plays  on  the  rubber  as  it  passes 
between  the  rolls,  and  as  fresh  surfaces  of  the  rubber  are  exposed,  the 
water  washes  out  the  impurities. 

The  finisher  is  a  mill  similar  to  the  refiner,  but  having  even  smaller 
corrugations  on  the  front  roll.  Here  the  rubber  is  sheeted  out  thin,  ready 
for  drying. 

Drying.  Two  processes  are  used  for  drying:  "air-drying"  and  "vacuum 
drying." 

In  the  air-drying,  the  Ions;  sheets  from  the  washing  mills  are  hung  over 
poles  in  a  drying  room,  the  temperature  of  which  is  maintained  at  90-110F. 

139 


140  THE  ANALYSIS  OF  RUBBER 

Fresh  air  is  kept  circulating  through  the  room,  so  as  to  facilitate  drying. 
In  the  plantation  rubbers,  the  moisture  is  usually  on  the  surface  only,  and 
such  rubbers  can  be  dried  in  from  two  to  four  days.  Many  of  the  wild 
rubbers  contain  a  large  amount  of  moisture,  and  take  up  a  further  quan- 
tity during  the  boiling  and  washing.  This  moisture  is  distributed  through- 
out the  rubber,  and  drying  takes  from  three  to  five  weeks. 

In  vacuum  drying,  the  rubber  is  laid  on  pans,  which  are  placed  in  a 
steam-heated  vacuum  oven.  The  process  requires  only  a  few  hours,  but 
it  is  not  particularly  well  adapted  for  drying  the  wild  rubbers,  since  the 
rapid  evaporation  tends  to  form  a  hard  surface  film,  which  hinders  further 
evaporation,  prolonging  the  drying  to  such  a  degree  as  to  materially  soften 
and  injure  the  rubber.  Soft  wild  rubbers,  such  as  Guayule,  Pontaniak, 
and  the  Africans,  flow  so  readily  at  ordinary  temperatures  that  they  can- 
not be  dried  on  poles,  and  must  be  dried  in  pans,  either  in  the  drying 
rooms  or  in  the  vacuum  dryers. 

Milling,  or  Breaking  Down.  Crude  rubber  varies  considerably  in  quality 
and  in  order  to  lessen  the  variation,  lots  of  rubber  are  averaged  by  the 
process  of  "milling"  or  "breaking  down."  For  this  purpose,  we  use  the 
regular  mixing  mill,  consisting  of  two  smooth  rolls,  the  rear  one  having 
a  higher  circumferential  speed  than  the  front  one.  The  rolls  are  hollow, 
so  as  to  permit  them  to  be  heated,  or  cooled,  in  order  that  they  may  be 
maintained  at  a  temperature  of  about  175F.  After  the  mills  have  been 
running  a  short  time,  the  friction  of  the  rubber  is  more  than  sufficient  to 
maintain  this  temperature,  and  cold  water  is  kept  running  through  them 
to  prevent  them  getting  so  hot  as  to  injure  the  rubber. 

The  rubber  is  fed  into  the  mill,  and  as  it  softens  it  adheres  to  the  front 
roll.  It  is  cut  off,  rolled  up,  and  fed  back  into  the  mill  until  it  is 
homogeneous.  By  this  time,  it  has  attained  a  soft,  wax-like  consistency. 
When  sufficient  uniformity  has  been  attained,  the  rubber  is  cut  off  in  the 
form  of  sheets,  or  rolls. 

This  "milling"  not  only  produces  a  uniform  grade  of  rubber,  but  it 
breaks  up  the  hard,  tough  particles  of  crude  rubber,  and  prepares  it  for  the 
following  operations  of  mixing,  calendaring,  and  tubing. 

Master  Batches.  Some  pigments,  and  many  of  the  organic  accelerators, 
are  first  mixed  with  rubber,  in  what  are  known  as  "master  batches,"  before 
they  are  sent  to  the  compounding  room  (where  the  batches  for  making 
the  various  rubber  compounds  are  weighed  out).  Pigments  such  as  glue, 
which  requires  a  longer  mixing  than  can  safely  be  given  the  entire  com- 
pound, are  first  mixed  with  rubber.  Glue,  for  example,  is  usually  mixed 
in  the  proportion  of  two  or  three  parts  of  rubber  to  one  of  glue.  Dusty 
pigments,  such  as  gas  black,  are  treated  in  a  similar  way,  frequently  in  a 
building  separated  from  the  regular  mixing  room.  This  step  is  necessary 
to  prevent  the  black  dust  settling  down  on  the  other  mills,  and  injuring 
the  colors  of  the  other  batches.  Organic  accelerators  are  used  in  Final  1 
amounts,  sometimes  0.2%  or  less.  It  is  highly  important  that  these  small 
amounts  be  very  evenly  distributed  throughout  the  entire  compound. 
After  these  accelerators  have  been  thoroughly  incorporated  into  a  master 
batch,  it  is  very  simple  to  distribute  them  throughout  a  compound.  A 
further  advantage  is  found  in  the  improved  accuracy  of  weighing  out  the 
small  amounts  needed,  and  elimination  of  losses  of  the  accelerator  during 
the  mixing.  Small  errors  or  losses  in  getting  the  accelerator  into  a  com- 
pound will  produce  more  variation  in  the  finished  product  than  in  any 
other  single  ingredient. 

Screening,  or  Bolting.  Foreign  matter,  such  as  wood,  grit,  pieces  of 
paper  or  twine  from  the  containers  in  which  the  pigments  are  shipped,  are 
objectionable  in  many  of  the  better  grades  of  rubber  articles.  Such 
impurities  are  removed  from  the  pigments  by  sifting  the  latter  through 
screens  of  from  40  to  90  mesh,  the  size  depending  largely  upon  the  pig- 
ment, and  the  article  in  which  it  is  to  be  used. 


APPENDIX  141 

Compounding.  The  compounding  consists  simply  in  weighing  out  the 
proper  amounts  of  pigments  and  rubbers.  These  are  placed  in  large  iron 
boxes  or  pans,  and  the  total  weight  checked  up  before  leaving  the  com- 
pound room.  The  usual  tolerance  in  weighing  is  0.5  Ib.  over  or  under,  in 
a  100  Ib.  batch,  or  a  total  variation  of  1%. 

Mixing.  The  mixing  mills  used  for  mixing  rubber  compounds  vary  from 
36  to  84  inches  in  length,  and  20  to  24  inches  in  diameter.  The  amount 
of  stock  that  can  be  mixed  on  a  mill  is  largely  a  function  of  the  width 
and  diameter  of  the  front  roll  of  the  mill,  and  the  specific  gravity  of  the 
stocks;  these  batches  will  range  from  25  to  225  Ibs. 

The  rubber  is  first  thrown  on  the  mill,  together  with  any  pigments  put 
up  in  master  batches,1  reclaimed  rubber,  and  mineral  rubber.  After  the 
rubber  softens,  the  pigments  and  oils  are  added.  Any  material  which 
drops  between  the  rolls  is  caught  in  a  pan,  and  returned  to  the  rolls  until 
everything  has  been  incorporated.  The  mixing  is  continued  until  the  com- 
pound is  homogeneous,  after  which  it  is  cut  off  from  the  rolls  in  slabs  of 
about  one  quarter  to  three-eighths  of  an  inch  in  thickness,  and  weighing 
15  to  25  Ibs.  each.  These  slabs  are  laid  on  racks  to  cool,  after  which  they 
are  sent  to  storage  bins  to  age  for  24  to  48  hours. 

Calendaring.  A  calendar  consists  of  three  smooth  hollow  steel  rolls, 
accurately  ground,  arranged  vertically,  the  top  and  bottom  rolls  revolving 
in  the  opposite  direction  to  the  middle  roll.  The  stock,  which  has  been 
previously  softened  or  "warmed-up"  on  the  regular  mixing  mill,  is  fed 
into  the  calendar  between  the  top  and  middle  rolls,  passes  around  the 
middle  and  under  the  bottom  roll,  after  which  it  is  wrapped  up  between 
cotton  liners.  The  function  of  these  liners  is  to  keep  the  fresh  surfaces 
of  the  rubber  apart,  and  protect  them  from  dust  and  dirt.  The  thickness 
of  the  sheet  is  controlled  by  the  distance  between  the  top  and  middle 
rolls;  the  width  by  adjustable  knives  placed  against  the  rear  of  the  middle 
rolls.  The  middle  and  bottom  rolls  are  further  apart  than  the  thickness 
of  the  rubber  sheet.  The  bottom  roll  is  cooled,  which  serves  to  cool 
the  rubber,  and  toughen  the  sheet,  before  it  is  wound  up  in  the  liner. 

The  rubber  sheet  may  be  made  of  the  desired  thickness  in  one  passing 
through  the  calendar,  or  it  may  be  built  up  from  a  number  of  thinner 
sheets. 

Calendaring  Rubber  to  Fabric.  When  a  rubber  compound  is  to  be 
applied  to  a  roll  of  fabric,  the  rubber  is  fed  into  the  calendar  as  above,  and 
the  fabric  is  fed  from  the  opposite  side,  between  the  middle  and  bottom 
rolls.  The  first  coat  is  put  on  by  having  the  middle  roll  rotate  at  a  faster 
speed  than  the  bottom  one.  giving  a  grinding  action  which  forces  the 
rubber  into  the  meshes  of  the  fabric.  This  is  called  "frictioning."  If  a 
further  layer  of  rubber  is  desired  over  the  friction  coat,  the  rubber  and 
fabric  are  fed  in  as  before,  but  the  middle  and  bottom  rolls  rotate  at  the 
same  speed,  and  the  sheet  of  rubber  is  thus  laid  on  the  fabric,  and  pressed 

1  We  have  here  an  added  advantage  in  using  organic  accelerators  in  the  form 
of  master  batches.  The  accelerator  is  uniformly  distributed  throughout 
the  rubber  before  any  of  the  sulfur  is  added,  thus  eliminating  largely  the 
danger  of  partial  vulcanization  on  the  mixing  mill  (called  burning,  or  scorch- 
ing). It  is  a  well  established  fact  that  high  concentrations  of  many  of  the 
organic  accelerators  greatly  increase  the  reaction  of  vulcanization,  especially 
at  the  lower  temperatures  existing  on  the  mixing  mill.  A  compound  containing 
0.50%  of  dimethyldithiocarbamate  burned  in  less  than  one  minute  after  the 
sulfur  has  been  added,  whereas  a  similar  stock,  having  only  0.05%,  mixed  with- 
out difficulty.  Andres  (Caoutchouc  &  Guttapercha  IS,  11089-97  [1921]),  showed 
that  2.5%  of  thiocarbanilide  gave  the  best  cure  at  50  minutes,  whereas  5% 
gave  a  good  cure  in  3  minutes,  and  the  best  cure  in  5  minutes.  These  higher 
concentrations  are  easily  obtained  in  a  poorly  mixed  stock,  and  obviously  in- 
crease the  probability  of  damage  to  the  stock. 


142  THE  ANALYSIS  OF  RUBBER 

together  just  sufficiently  for  them  to  adhere  firmly.  This  coating  of  rubber 
is  called  the  "skim"  coat;  tire  fabric,  for  example,  is  usually  frictioned  on 
both  sides,  and  skim-coated  on  one  side  only.  After  the  skim  has  been 
applied  the  fabric  is  rolled  up  between  a  liner,  to  be  cut  up  later  as  desired. 

Tubing.  The  tubing  machine  is  used  for  irregular  strips  of  rubber,  such 
as  tire  treads,  tire  beads,  rubber  tubing,  and  the  insulation  on  wire.  The 
essential  parts  of  a  tubing  machine  are  a  hopper  for  feeding  in  the  stock, 
the  barrel,  which  can  be  heated  by  steam,  and  which  contains  the  screw 
for  carrying  the  stock  to  the  head,  and  the  head,  containing  the  die  through 
which  the  stock  is  forced.  The  head  is  often  heated  with  a  gas  jet,  to 
prevent  the  tubed  rubber  cooling  in  the  die  and  coming  through  rough  or 
cracked.  The  rubber  is  warmed  up  to  the  desired  consistency,  cut  into 
small  strips,  and  fed  into  the  hopper.  The  stock  as  it  comes  from  the 
tubing  machine  is  cut  in  the  desired  lengths,  placed  between  liners,  and 
set  aside  to  cool. 

'Cement..  In  the  manufacture  of  cement,  the  rubber  is  warmed  up  on 
the  mixing  mill,  and  cut  off  in  very  thin  sheets.  A  weighed  amount  of 
this  rubber  is  cut  into  small  pieces,  and  thrown  into  a  churn  or  mixer, 
to  which  has  already  been  added  the  measured  amount  of  solvent.  The 
contents  of  the  mixer  are  stirred  until  solution  is  complete,  a  matter  of 
from  4  to  12  hours,  depending  largely  upon  the  nature  of  the 
solvent,  the  grade  of  rubber,  and  the  efficiency  of  the  mixer.  The  first 
mixing  usually  gives  a  heavier  cement  than  desired,  and  this  is  thinned 
down  with  more  solvent  until  the  right  viscosity  is  obtained.  The  nature 
of  the  service  for  which  the  cement  is  intended  dictates  the  degree  of 
viscosity. 

Spreading.  In  addition  to  the  method  of  applying  rubber  to  fabric  by 
calendaring,  as  described  above,  we  may  use  the  process  known  as  "spread- 
ing." A  spreader  consists  of  a  rubber  coated  roll,  against  which  rests  a 
heavy  knife.  Beyond  the  rubber  roll  are  steam-heated  coils  or  plates, 
about  18  to  30  feet  in  length.  The  rubber  compound  is  first  made  into 
a  very  heavy  cement  (generally  called  "dough").  The  fabric  is  passed 
between  the  rubber-coated  roll  and  the  knife,  the  dough  is  applied  to  the 
fabric  just  before  the  latter  reaches  the  knife,  and  in  passing  between  the 
roll  and  the  knife  the  latter  scrapes  off  all  but  a  thin  coating  of  the  cement. 
As  the  fabric  passes  over  the  heated  plates,  the  solvent  in  the  cement 
evaporates,  and  leaves  a  thin  coating  of  rubber  in  very  intimate  contact 
with  the  fabric.  The  space  between  the  knife  and  roll  controls  the  amount 
of  rubber  left  upon  the  fabric.  The  amount  of  rubber  which  may  be  added 
at  one  passing  depends  upon  the  ability  of  the  spreader  to  drive  off  the 
solvent  during  the  time  when  the  fabric  is  passing  over  the  heated  rolls. 
The  factors  are  the  temperature  and  length  of  the  drying  plates,  and  the 
speed  at  which  the  machine  is  driven.  A  heavy  coating  of  rubber  is  ob- 
tained by  passing  the  fabric  through  until  the  desired  quantity  of  rubber 
has  been  applied. 


APPENDIX  B. 

PHYSICAL  TESTS. 

The  chemist  in  the  rubber  factory  is  usually  given  the  duty  of  making 
whatever  physical  tests  may  be  necessary  to  determine  the  properties  of 
the  rubber  compound  or  finished  article.  Similarly,  the  chemist  in  the 
consumer's  laboratory  supervises  and  interprets  the  results  of  the  physical 
tests  made  upon  samples  taken  from  deliveries  of  manufactured  goods. 
It  seems  desirable,  therefore,  to  point  out  what  physical  tests  are  usually 
made,  and  their  relation  to  the  quality  and  life  of  the  material. 

The  principal  physical  tests  are  (1)  tensile  strength,  (2)  ultimate  elonga- 
tion, (3)  set  at  break,  (4)  friction.  These  tests  are  usually  made  on  the 
same  testing  machine.  The  tensile  strength  is  the  force  required  to  break 
a  unit  area  of  a  rubber  compound;  the  ultimate  elongation  is  the  extent 
to  which  the  rubber  can  be  stretched  before  it  will  break;  the  set  at 
break  is  the  increase  in  length  of  a  measured  length  of  rubber,  taken  at 
some  definite  time  after  break;  and  the  friction  is  the  force  required  to 
separate  a  rubber  compound  from  .a  piece  of  fabric  to  which  it  has  been 
vulcanized. 

Tensile  Testing  Machine.  Three  types  of  machines  are  in  more  or  less 
common  use  in  this  country:  (a)  Scott;  (b)  Bureau  of  Standards;  (c) 
Schopper. 

(a)  The  Scott  is  a  machine  of  the  dead  weight  type,  the  pull  being 
against  a  lever  which  moves  outward  as  the  tension  is  applied.  There 
are  two  clamps,  into  which  are  inserted  the  ends  of  the  rubber  test  pieces. 
The  upper  clamp  is  attached  to  the  end  of  the  weighted  lever;  the  lower 
clamp  is  attached  to  a  rod  driven  at  a  uniform  rate  of  speed  (usually  20 
inches  per  minute  for  tensile  tests,  and  2  inches  per  minute  for  friction 
tests).  The  lever  carries  a  set  of  pawls,  which  engage  in  the  teeth  of  a 
curved  rack,  preventing  the  lever  from  falling  back  when  the  tension  is 
released  (as  for  example,  when  the  test  piece  breaks) .  The  tension  is  read 
off  from  a  dial,  the  indicator  being  actuated  by  the  motion  of  the  lever. 

(6)  The  Bureau  of  Standards  machine1  differs  from  the  Scott  in  that 
the  pull  is  against  a  spring  balance,  which  directly  records  the  pull.  The 
clamps  are  of  the  same  type,  and  the  operation  of  the  machine  is  essen- 
tially the  same  as  that  of  the  Scott. 

(c)  The  Schopper  machine  is  one  of  the  dead  weight  type.  The  rack, 
over  which  the  lever  moves,  is  graduated,  and  the  tension  is  read  off  oppo- 
site the  point  where  the  lever  stops. 

There  are  a  number  of  styles  of  clamps  which  may  be  used  with  these 
machines,  the  principal  ones  being  the  eccentric  grip,  with  its  modification 
consisting  of  a  number  of  thin  disks,  mounted  eccentrically,  the  zig-zag 
grip,  which  is  tightened  by  a  screw,  and  the  spool  grips,  for  use  with 
ring-shaped  test  pieces.  Any  of  these  types  may  be  used  with  any  of  the 
machines  mentioned,  but  the  Scott  and  the  Bureau  of  Standards  machines 
usually  carry  the  eccentric  grip  clamps,  for  testing  bar-shaped  test  pieces, 
whereas  the  Schopper  usually  has  only  the  spool  grips.  As  long  as  the 
Schopper  is  equipped  only  with  grips  for  testing  ring-shaped  test  pieces, 


Cf.  Bureau  of  Standards  Circular  38,  Fourth  Edition,  p.  53, 

143 


144  THE  ANALYSIS  OF  RUBBER 

it  cannot  be  considered  equivalent  to  the  other  machines.  Between  the 
Scott  and  the  Bureau  of  Standards  machines  there  is  little  choice  to  be 
made — providing  they  are  both  accurately  calibrated,  and  the  clamps  sep- 
arated at  the  same  rate  of  speed,  comparable  results  may  be  obtained. 
The  dead  weight  type  is  usually  considered  to  be  the  more  rugged,  and 
less  likely  to  get  out  of  order,  than  the  spring  balance. 

When  using  the  ring-shaped  test  pieces,  any  one  of  the  three  machines 
may  be  used  without  affecting  the  results.  In  fact,  the  type  of  machine 
is  of  importance,  not  so  much  for  the  accuracy  of  the  determinations  which 
it  will  give,  but  from  the  point  of  view  as  to  how  it  will  stand  up  under 
the  service  given  to  it,  and  the  convenience  of  operation. 

Shape  of  Test  Pieces.  The  test  piece  commonly  used  in  this  country  for 
the  determination  of  tensile  strength,  is  the  "bar-shaped"  or  "dumb-bell" 
test  piece.  The  constricted  part  is  either  }4  or  %"  and  1  or  2"  long.  The 
ends  are  enlarged  to  reduce  to  the  minimum  the  danger  of  the  test  piece 
tearing  in  the  clamps.  The  enlarged  ends  are  I"  wide  for  the  %"  width, 
and  1%''  wide  for  the  %"  width.  A  few  use  a  W  width  at  the  ends  for 
a  *4"  width  at  the  constricted  part,  particularly  for  testing  compounds  of 
high  rubber  content  (the  so-called  pure  gum  compounds).  The  style  of 
test  piece  is  largely  a  matter  of  the  operator's  choice,  influenced  in  part 
by  the  nature  of  the  material  to  be  tested.  Some  specifications  define 
exactly  the  shape,  leaving  nothing  to  the  discretion  of  the  operator.  While 
theoretically  there  should  be  no  difference,  as  a  matter  of  practice  results 
are  comparable  only  when  the  same  shaped  test  piece  is  used. 

In  the  above,  nothing  has  been  said  regarding  the  thickness  of  the  test 
piece.  Except  when  slabs  are  prepared  particularly  for  the  purpose  of 
making  tensile  tests,  this  is  not  a  matter  which  can  be  controlled  easily, 
but  the  thickness  is  quite  likely  to  be  an  important  factor,  the  thicker 
pieces  showing  a  greater  tendency  to  tear,  and  hence  giving  lower  results 
than  would  be  obtained  from  thinner  ones.  The  most  satisfactory  prac- 
tical range  is  from  %  to  3/16"  (0.125  to  0.183"). 

^The  ring-shaped  test  piece  cannot  be  compared  with  the  bar-shaped  test 
piece.*  Its  only  advantage  lies  in  the  fact  that  with  it  an  autographic 
chart  may  be  made  of  the  stress-strain  curve.  With  the  bar-shaped  test 
piece,  to  get  the  same  data,  it  is  necessary  to  use  two  operators,  and  from 
their  observations  plot  the  stress-strain  curve. 

The  principal  precautions  to  be  taken  in  preparing  test  pieces  of  any 
shape  are  that  the  edges  be  cut  evenly  and  that  the  opposite  sides  of 
the  constricted  part  are  parallel.  With  ring-shaped  test  pieces,  the  rings 
must  be  very  accurately  centered,  so  as  to  obtain  the  same  cross-section 
at  all  points.  If  the  top  and  bottom  surfaces  of  the  test  pieces  are  not 
smooth,  they  should  be  made  so  by  buffing,  so  that  accurate  readings  of 
thickness  may  be  made.8 

Tensile  Strength.  The  tensile  strength  is  usually  expressed  in  pounds 
per  square  inch,  or  kilograms  per  square  centimeter.4  The  area  is  usually 
referred  to  the  cross-section  at  rest.  However,  before  rubber  can  be 
broken,  it  must  be  stretched  from  300  to  900%,  and  since  there^  is  no 
change  in  volume 5  during  the  stretching,  the  cross-section  at  break  is  very 
much  less  than  when  the  test  piece  is  at  rest.  For  this  reason,  the  tensile 

•This  point  is  argued  very  convincingly  in  the  Bureau  of  Standards  Circular 
38,  Fourth  Edition,  p.  66,  etc. 

•A  buffing  machine,  suitable  for  the  purpose,  is  described  in  Bureau  of  Stand- 
ards Circular  No.  38,  Fourth  Edition,  p.  48. 

4  To  convert  Ibs.  /  sq.  in.  into  kg.  /  sq.  cm.,  multiply  by  0.07031 ;  to  convert 
kg.  /  sq.  cm.  to  Ibs.  /  sq.  in.,  multiply  by  14.222. 

•  Schippel's  change  in  volume  on  stretching  refers  only  to  the  vacua  formed 
around  coarse  particles  of  pigment.  Such  changes  are  negligible  for  the  calcu- 
lations under  discussion. 


APPENDIX  145 

strength  has  sometimes  been  referred  to  the  cross-section  at  break,  called 

the  "tensile  product."    This  figure  is  obtained  by  multiplying  the  tensile 
strength  by  the  elongation  at  break. 

The  tensile  strength  is  appreciably  affected  by  a  considerable  number 
of  factors,  some  of  which  are  within  the  control  of  the  operator  and  some 
are  not.  Of  these,  the  most  important  are:  rate  of  separation  of  the  jaws, 
temperature,  size  and  shape  of  the  test  pieces,  the  direction  of  the  cut 
(i.e.,  whether  along  the  length  of  a  calendared  sheet,  or  across),  previous 
stretching  of  the  rubber,  and  the  age  of  the  rubber  compound.  These 
factors  have  been  discussed  at  some  length  by  Whitby,6  and  the  Bureau 
of  Standards,7  and  their  conclusions  may  be  briefly  summarized  as 
follows : 

Rate  of  Separation  of  the  Jaws.  The  higher  the  speed,  the  higher  will 
be  the  results  for  tensile  strength  and  ultimate  elongation.  The  range  in 
speeds  between  5"  and  45"  per  minute  may  affect  the  results  anywhere 
from  5%  to  20%.  The  speed 'generally  employed  is  20"  per  minute. 

Temperature.  The  temperature  at  which  tests  are  usually  expected  to 
be  made  is  70F.  Increasing  the  temperature  lowers  the  tensile  strength 
and  increases  the  elongation;  lowering  the  temperature  produces  a  reverse 
effect.  It  is  worthy  of  notice  that  for  the  range  of  temperature  from  50F 
to  90F,  the  tensile  at  break  is  much  more  constant  than  either  the  tensile 
strength  or  ultimate  elongation. 

Size  and  Shape  of  the  Test  Pieces.  There  is  a  tendency  for  narrow  test 
pieces  to  develop  higher  values  than  wider  ones;  between  W  and  W, 
differences  as  high  as  20%  have  been  noted.  Unpublished  data  may  con- 
tain instances  of  even  greater  variation. 

Direction  of  Cutting^.  The  Bureau  of  Standards  found  that  the  tests 
made  on  samples  cut  in  the  direction  of  calendaring  show  a  higher  ten- 
sile and  lower  elongation  than  those  cut  in  the  transverse  direction. 
Some  experimenters  have  not  been  able  to  duplicate  these  results,  but 
most  of  the  data  on  the  subject  indicate  that  there  is  a  decided  difference 
between  the  two  directions  in  a  calendared  sheet.  The  ring-shaped  test 
pieces  include  rubber  cut  in  all  directions,  and  since  the  break  occurs  in 
the  direction  of  least  resistance,  it  is  obvious  that  the  effects  of  calendaring 
cannot  be  detected  with  such  test  pieces. 

Previous  Stretching  of  the  Rubber  Test  Pieces.  It  is  curious  that  while 
Memmler  and  Schob,  and  the  Bureau  of  Standards,  agree  that  previous 
stretching  alters  the  results  of  tensile  tests,  the  former  obtained  lower 
results  from  test  pieces  subjected  to  previous  stretching,  whereas  the  latter 
obtained  higher  figures.  Memmler  and  Schob  tested  ring-shaped  test 
pieces  by  subjecting  them  to  50%  of  their  normal  breaking  load  for  a 
period  of  30  minutes,  and  testing  them  after  a  rest  period  of  24  hours. 
The  Bureau  of  Standards  employed  bar-shaped  test  pieces,  stretching  to 
200%,  releasing,  and  then  continuing  with  an  increase  of  100%  until  failure 
ensued.  With  high  grade  material,  Memmler  and  Schob  found  differences 
of  about  35%  loss  in  tensile  strength;  the  Bureau  of  Standards  found  in- 
creases of  about  20%.  Apparently  what  is  needed  to  determine  the  exact 
difference  caused  by  previous  stretching,  is  to  combine  the  two  sets  of 
ideas.  The  Bureau  of  Standards  figures  seem  to  show  that  short  periods 
of  stretching  increase  the  tensile  strength.  By  increasing  the  time  of 
stressing  the  test  pieces,  we  could  find  out  whether  or  not  there  is  a  point 
at  which  there  is  no  further  increase  in  tensile  properties.  Similar  experi- 
ments could  be  made  with  ring-shaped  test  pieces,  for  it  is  not  at  all  im- 
possible that  the  differences  may  be  largely  attributed  to  the  differences 
in  the  shape  of  the  test  pieces. 
The  greatest  importance  of  these  experiments  lies  in  the  fact  that  they 

•  Whitby,  Plantation  rubber  and  the  testing  of  rubber. 
'Bureau  of  Standards  Circular  38,  Fourth  Edition, 


144  THE  ANALYSIS  OF  RUBBER 

it  cannot  be  considered  equivalent  to  the  other  machines.  Between  the 
Scott  and  the  Bureau  of  Standards  machines  there  is  little  choice  to  be 
made — providing  they  are  both  accurately  calibrated,  and  the  clamps  sep- 
arated at  the  same  rate  of  speed,  comparable  results  may  be  obtained. 
The  dead  weight  type  is  usually  considered  to  be  the  more  rugged,  and 
less  likely  to  get  out  of  order,  than  the  spring  balance. 

When  using  the  ring-shaped  test  pieces,  any  one  of  the  three  machines 
may  be  used  without  affecting  the  results.  In  fact,  the  type  of  machine 
is  of  importance,  not  so  much  for  the  accuracy  of  the  determinations  which 
it  will  give,  but  from  the  point  of  view  as  to  how  it  will  stand  up  under 
the  service  given  to  it,  and  the  convenience  of  operation. 

Shape  of  Test  Pieces.  The  test  piece  commonly  used  in  this  country  for 
the  determination  of  tensile  strength,  is  the  "bar-shaped"  or  "dumb-bell" 
test  piece.  The  constricted  part  is  either  %  or  %"  and  1  or  2"  long.  The 
ends  are  enlarged  to  reduce  to  the  minimum  the  danger  of  the  test  piece 
tearing  in  the  clamps.  The  enlarged  ends  are  I"  wide  for  the  W  width, 
and  1%"  wide  for  the  %"  width.  A  few  use  a  VA"  width  at  the  ends  for 
a  %"  width  at  the  constricted  part,  particularly  for  testing  compounds  of 
high  rubber  content  (the  so-called  pure  gum  compounds).  The  style  of 
test  piece  is  largely  a  matter  of  the  operator's  choice,  influenced  in  part 
by  the  nature  of  the  material  to  be  tested.  Some  specifications  define 
exactly  the  shape,  leaving  nothing  to  the  discretion  of  the  operator.  While 
theoretically  there  should  be  no  difference,  as  a  matter  of  practice  results 
are  comparable  only  when  the  same  shaped  test  piece  is  used. 

In  the  above,  nothing  has  been  said  regarding  the  thickness  of  the  test 
piece.  Except  when  slabs  are  prepared  particularly  for  the  purpose  of 
making  tensile  tests,  this  is  not  a  matter  which  can  be  controlled  easily, 
but  the  thickness  is  quite  likely  to  be  an  important  factor,  the  thicker 
pieces  showing  a  greater  tendency  to  tear,  and  hence  giving  lower  results 
than  would  be  obtained  from  thinner  ones.  The  most  satisfactory  prac- 
tical range  is  from  %  to  3/16"  (0.125  to  0.183"). 

^The  ring-shaped  test  piece  cannot  be  compared  with  the  bar-shaped  test 
piece.*  Its  only  advantage  lies  in  the  fact  that  with  it  an  autographic 
chart  may  be  made  of  the  stress-strain  curve.  With  the  bar-shaped  test 
piece,  to  get  the  same  data,  it  is  necessary  to  use  two  operators,  and  from 
their  observations  plot  the  stress-strain  curve. 

The  principal  precautions  to  be  taken  in  preparing  test  pieces  of  any 
shape  are  that  the  edges  be  cut  evenly  and  that  the  opposite  sides  of 
the  constricted  part  are  parallel.  With  ring-shaped  test  pieces,  the  rings 
must  be  very  accurately  centered,  so  as  to  obtain  the  same  cross-section 
at  all  points.  If  the  top  and  bottom  surfaces  of  the  test  pieces  are  not 
smooth,  they  should  be  made  so  by  buffing,  so  that  accurate  readings  of 
thickness  may  be  made.8 

Tensile  Strength.  The  tensile  strength  is  usually  expressed  in  pounds 
per  square  inch,  or  kilograms  per  square  centimeter.4  The  area  is  usually 
referred  to  the  cross-section  at  rest.  However,  before  rubber  can  be 
broken,  it  must  be  stretched  from  300  to  900%,  and  since  there^  is  no 
change  in  volume B  during  the  stretching,  the  cross-section  at  break  is  very 
much  less  than  when  the  test  piece  is  at  rest.  For  this  reason,  the  tensile 

•This  point  is  argued  very  convincingly  in  the  Bureau  of  Standards  Circular 
38,  Fourth  Edition,  p.  66,  etc. 

»A  buffing  machine,  suitable  for  the  purpose,  is  described  in  Bureau  of  Stand- 
ards Circular  No.  38,  Fourth  Edition,  p.  48. 

*  To  convert  Ibs.  /  sq.  in.  into  kg.  /  sq.  cm.,  multiply  by  0.07031 ;  to  convert 
kg.  /  sq.  cm.  to  Ibs.  /  sq.  in.,  multiply  by  14.222. 

•  Schippel's  change  in  volume  on  stretching  refers  only  to  the  vacua  formed 
around  coarse  particles  of  pigment.     Such  changes  are  negligible  for  the  calcu- 
lations under  discussion. 


APPENDIX  145 

strength  has  sometimes  been  referred  to  the  cross-section  at  break,  called 
the  "tensile  product."  This  figure  is  obtained  by  multiplying  the  tensile 
strength  by  the  elongation  at  break. 

The  tensile  strength  is  appreciably  affected  by  a  considerable  number 
of  factors,  some  of  which  are  within  the  control  of  the  operator  and  some 
are  not.  Of  these,  the  most  important  are:  rate  of  separation  of  the  jaws, 
temperature,  size  and  shape  of  the  test  pieces,  the  direction  of  the  cut 
(i.e.,  whether  along  the  length  of  a  calendared  sheet,  or  across),  previous 
stretching  of  the  rubber,  and  the  age  of  the  rubber  compound.  These 
factors  have  been  discussed  at  some  length  by  Whitby,6  and  the^Bureau 
of  Standards,7  and  their  conclusions  may  be  briefly  summarized  as 
follows : 

Rate  of  Separation  of  the  Jaws.  The  higher  the  speed,  the  higher  will 
be  the  results  for  tensile  strength  and  ultimate  elongation.  The  range  in 
speeds  between  5"  and  45"  per  minute  may  affect  the  results  anywhere 
from  5%  to  20%.  The  speed 'generally  employed  is  20"  per  minute. 

Temperature.  The  temperature  at  which  tests  are  usually  expected  to 
be  made  is  70F.  Increasing  the  temperature  lowers  the  tensile  strength 
and  increases  the  elongation;  lowering  the  temperature  produces  a  reverse 
effect.  It  is  worthy  of  notice  that  for  the  range  of  temperature  from  50F 
to  OOF,  the  tensile  at  break  is  much  more  constant  than  either  the  tensile 
strength  or  ultimate  elongation. 

Size  and  Shape  of  the  Test  Pieces.  There  is  a  tendency  for  narrow  test 
pieces  to  develop  higher  values  than  wider  ones;  between  *4"  and  %", 
differences  as  high  as  20%  have  been  noted.  Unpublished  data  may  con- 
tain instances  of  even  greater  variation. 

Direction  of  Cutting.  The  Bureau  of  Standards  found  that  the  tests 
made  on  samples  cut  in  the  direction  of  calendaring  show  a  higher  ten- 
sile and  lower  elongation  than  those  cut  in  the  transverse  direction. 
Some  experimenters  have  not  been  able  to  duplicate  these  results,  but 
most  of  the  data  on  the  subject  indicate  that  there  is  a  decided  difference 
between  the  two  directions  in  a  calendared  sheet.  The  ring-shaped  test 
pieces  include  rubber  cut  in  all  directions,  and  since  the  break  occurs  in 
the  direction  of  least  resistance,  it  is  obvious  that  the  effects  of  calendaring 
cannot  be  detected  with  such  test  pieces. 

Previous  Stretching  of  the  Rubber  Test  Pieces.  It  is  curious  that  while 
Memmler  and  Schob,  and  the  Bureau  of  Standards,  agree  that  previous 
stretching  alters  the  results  of  tensile  tests,  the  former  obtained  lower 
results  from  test  pieces  subjected  to  previous  stretching,  whereas  the  latter 
obtained  higher  figures.  Memmler  and  Schob  tested  ring-shaped  test 
pieces  by  subjecting  them  to  60%  of  their  normal  breaking  load  for  a 
period  of  30  minutes,  and  testing  them  after  a  rest  period  of  24  hours. 
The  Bureau  of  Standards  employed  bar-shaped  test  pieces,  stretching  to 
200%,  releasing,  and  then  continuing  with  an  increase  of  100%  until  failure 
ensued.  With  high  grade  material,  Memmler  and  Schob  found  differences 
of  about  35%  loss  in  tensile  strength;  the  Bureau  of  Standards  found  in- 
creases of  about  20%.  Apparently  what  is  needed  to  determine  the  exact 
difference  caused  by  previous  stretching,  is  to  combine  the  two  sets  of 
ideas.  The  Bureau  of  Standards  figures  seem  to  show  that  short  periods 
of  stretching  increase  the  tensile  strength.  By  increasing  the  time  of 
stressing  the  test  pieces,  we  could  find  out  whether  or  not  there  is  a  point 
at  which  there  is  no  further  increase  in  tensile  properties.  Similar  experi- 
ments could  be  made  with  ring-shaped  test  pieces,  for  it  is  not  at  all  im- 
possible that  the  differences  may  be  largely  attributed  to  the  differences 
in  the  shape  of  the  test  pieces. 

The  greatest  importance  of  these  experiments  lies  in  the  fact  that  they 

6  Whitby,  Plantation  rubber  and  the  testing  of  rubber. 
*  Bureau  of  Standards  Circular  38,  Fourth  Edition, 


146  THE  ANALYSIS  OF  RUBBER 

emphasize  the  necessity  for  permitting  rubber  samples  to  age  for  at  least 
48  hours,  in  order  to  be  certain  that  they  have  reached  equilibrium. 

Aging  of  the  Rubber.  Practically  every  one  who  has  followed  the  test- 
ing of  rubber  is  agreed  that  a  certain  time  is  required  after  vulcanization 
for  the  rubber  to  come  to  equilibrium.  To  be  absolutely  safe,  many  have 
placed  the  period  for  aging  at  3  days;  others  think  that  as  little  as  24 
hours  will  suffice.  In  view  of  the  results  obtained  in  a  study  of  the  effects 
of  previous  stretching,  24  hours  seems  hardly  enough  to  be  on  the  safe 
side,  and  consequently  a  minimum  of  48  hours  is  recommended." 

Ultimate  Elongation.  The  ultimate  elongation  has  been  denned  as  being 
the  extent  to  which  a  rubber  compound  may  be  extended  before  rupture 
will  occur.  With  bar  test  pieces,  the  elongation  is  determined  by  placing 
on  the  constricted  portion  of  the  test  piece,  parallel  lines  either  1"  or  2" 
apart,  and  then  stretching  until  it  breaks.  The  distance  between  the 
marks  at  break  (o)  less  the  original  distance  (b)  is  the  elongation  (c), 
and  is  expressed  in  percentage.  Some  prefer  to  express  the  ultimate 
elongation  by  dividing  a  by  b,  giving  figures  which  are  100%  higher  than 
the  more  commonly  accepted  figures.  There  is  nothing  gained  by  this 
procedure,  and  it  causes  a  great  deal  of  confusion  when  making  compari- 
sons. In  order  to  avoid  this,  many  specifications  are  now  being  written 
calling  for  an  elongation  in  definite  figures,  such  as  from  1-5  inches,  or  6 
inches,  or  whatever  length  may  be  desired. 

For  correctly  cured  soft  vulcanized  rubber,  the  ultimate  elongation  is 
affected  most  by  the  amount  and  grade  of  rubber  present.  Compounds 
containing  90%  of  rubber  will  have  an  elongation  of  about  900%  to 
1000%,  while  compounds  containing  only  30%  will  have  an  elongation  of 
only  300%  to  500%.  Just  as  in  the  case  of  tensile  strength,  we  find  that  the 
ultimate  elongation  is  a  more  or  less  arbitrary  figure,  the  value  of  which 
will  depend  to  a  considerable  degree  upon  the  manner  of  its  determina- 
tion. Practically  all  of  the  factors  which  influence  the  values  for  tensile 
strength  will  be  found  to  have  an  effect  on  those  for  elongation. 

Stress-Strain  Curves.  If  the  tensile  strength  for  each  increment  of  elonga- 
tion be  determined,  and  plotted,  the  line  drawn  through  these  points 
gives  us  what  is  known  as  the  "stress-strain"  curve.  Generally  the  strains 
are  plotted  as  ordinates,  and  the  stresses  as  abscissae.  The  Scott  and 
Schopper  machines  plot  these  curves  autographically  when  ring-shaped  test 
pieces  are  used.  With  bar  test  pieces,  one  operator  reads  the  strains,  and 
the  other  the  stresses.  By  plotting  the  curves  of  a  series  of  cures  on  one 
sheet,  the  effect  of  time  of  vulcanization,  or  whatever  other  factor  it  is 
desired  to  follow,  may  be  easily  observed. 

The  principal  trouble  with  the  stress-strain  curve  for  rubber,  is  that  in 
making  the  curves  of  a  series  of  cures,  the  first  half  or  three-fourths  of 
the  curves  take  practically  the  same  course,  and  it  is  difficult,  if  not  impos- 
sible, to  notice  any  appreciable  difference  until  the  last  quarter  of  the 
curve.  The  Goodyear  laboratory  has  suggested  a  means  for  plotting  the 
results  in  such  a  fashion  as  to  bring  out  differences  in  the  early  parts 
of  the  curves.  They  plot  stresses  as  ordinates,  and  the  time  of  cure  as 
abscissae;  for  each  time  of  cure,  they  plot  the  tensile  strength  for  100% 
and  each  succeeding  100%  elongation  up  to  the  break.  Curves  are  drawn 
through  all  points  having  the  same  elongation,  and  the  final  curve  is  drawn 

•It  is  obvious  that  at  times  these  precautions  must  yield  to  expediency,  and 
it  is  frequently  more  important  in  manufacturing  work  to  get  immediate  results 
which  are  approximately  accurate,  than  to  wait  48  or  even  24  hours.  We  have 
frequently  taken  slabs  of  rubber  out  of  a  mold,  cooled  them  in  running  water  for 
15  minutes,  and  then  proceeded  with  the  tests.  In  all  such  cases,  the  proba- 
bility of  large  errors  being  present  was  known  and  appreciated.  Those  samples 
which  showed  any  promise  of  being  satisfactory  were  given  the  regular  tests 
48  hours  later,  and  the  latter  figures  only  were  used  for  record  and  comparison. 


APPENDIX  147 

through  the  points  of  ultimate  elongation.  The  latter  is  the  usual  "tensile- 
strength-time-of-cure"  curve  used  so  much  by  investigators  in  this  coun- 
try. This  system  of  plotting  gives  a  much  more  satisfactory  picture  than 
does  the  ordinary  stress-strain  curve. 

Set  at  Break.  After  the  test  piece  has  been  broken  on  the  testing 
machine  it  is  laid  aside  for  a  period  which  ranges  from  1  to  24  hours, 
according  to  the  methods  adopted  by  the  various  laboratories,  and  the 
increase  in  the  distance  between  the  marks  is  measured,  and  calculated 
to  percentage.  The  set  at  break  for  various  cures  of  the  same  compound 
passes  through  a  maximum  at  the  optimum  cure,  the  shape  of  the  curve 
as  plotted  against  time  of  cure  very  much  resembles  the  tensile  strength- 
time-of-cure  curve.  Very  little  practical  use  is  made  of  this  determination. 

A  far  more  extensive  use  of  the  determination  of  set  has  been  made 
by  determining  the  set  on  test  pieces  which  have  been  stretched  to  less 
than  their  ultimate  elongation.  The  usual  routine  in  such  tests  is  to 
stretch  the  test  piece  for  ten  minutes 9  and  measure  the  increase  in  elonga- 
tion ten  minutes  after  releasing.  With  'such  tests,  there  is  a  drop  in  the 
value  of  the  set  from  an  undercure  to  an  overcure,  the  effect  being  most 
noticeable  in  the  former. 

Friction.  The  adhesion  between  fabric  and  rubber  is  termed  "friction." 
There  are  two  methods  for  its  determination:  (a)  the  amount  of  separa- 
tion under  definite  load;  (b)  the  load  required  to  separate  rubber  and 
fabric  at  a  definite  rate. 

The  first  method  is  much  employed  in  testing  mechanical  goods.  In 
testing  belting,  for  example,  a  test  piece  is  cut  one  inch  wide  and  about 
six  inches  long.  Two  plies  of  fabric  are  separated;  the  end  of  one  ply  is 
mounted  in  a  rigid  position,  while  to  the  other  ply  is  attached  a  weighted 
clamp.  A  mark  is  made  where  the  test  is  to  start,  and  the  weighted  clamp 
is  then  released.  After  a  fixed  time  (generally  10  minutes),  the  amount 
of  separation  is  measured.  This  test  merely  gives  a  minimum  value,  and 
does  not  measure  the  true  adhesion. 

In  the  second  method,  one  ply  is  fastened  to  the  upper  clamp  of  a  test- 
ing machine,  and  the  other  ply  to  the  lower  one.  The  clamps  are  now 
separated  at  a  uniform  rate,  usually  2"  per  minute,  and  an  auto- 
graphic record  made  of  the  pull  required  to  separate  the  two  plies.  This 
method  not  only  shows  the  maximum  strength  of  the  adhesion,  but  gives 
the  variation  over  the  area  tested,  thus  revealing  any  lack  of  uniformity. 

Ordinarily,  the  adhesion  between  two  rubber  plies  cannot  be  tested  in 
this  manner,  since  the  joint  is  usually  stronger  than  either  of  the  two 
compounds.  However,  the  second  method  is  available  for  testing  adhesions 
such  as  are  found  in  the  acid  cured  splice  of  an  inner  tube,  or  between  a 
hard  rubber  and  a  soft  rubber  compound. 

Heat  Aging  Tests.  All  of  the  tests  described  above  are  performed  on 
test  pieces  in  the  condition  as  they  come  from  the  vulcanizers.  Such  tests 
give  little,  if  any,  idea  of  what  the  physical  properties  of  the  compound 
will  be  at  some  future  time.  Artificial  means  for  aging  have  been  sug- 
gested, but  probably  the  most  widely  used  is  the  one  developed  by  W.  C. 
Geer.10  The  test  pieces  are  maintained  at  a  constant  temperature  of  70C, 
and  tests  made  at  regular  intervals  until  deterioration  sets  in.  The  most 
serious  fault  with  this  method  is  that  the  heating  takes  place  in  air,  so 
that  in  addition  to  the  deterioration  caused  by  heating,  we  have  the  effect 
of  the  oxygen  of  the  air.  In  a  great  many  cases,  such  as  tire  frictions, 
breaker  stocks,  cushion  stocks,  etc.,  heat  is  undoubtedly  the  principal 
agency  in  deterioration.  In  such  cases,  the  heating  should  be  done  in  an 

•Cf.  Bureau  of  Standards  Circular  38,  Fojirth  Edition,  pages  57-8,  for  a 
description  of  a  convenient  form  of  apparatus  for  making  these  tests. 

10  W.  C.  Geer,  India  Rubber  World  55,  127-30  (1916)  :  W.  C.  Geer  and  W.  W. 
Evans,  India  Rubber  World  64,  887-92  (1921). 


148  THE  ANALYSIS  OF  RUBBER 

atmosphere  of  gas  free  from  oxygen,  or  any  gas  which  may  have  a  tendency 
to  react  with  rubber. 

The  most  which  can  be  claimed  for  this  test  is  that  it  is  useful  in  com- 
paring compounds  of  about  the  same  type,  or  of  cures  of  the  same  com- 
pound. With  oxygen  excluded,  it  might  be  extended  to  include  compounds 
of  different  types,  which  are  likely  to  be  exposed  to  the  same  degree  of 
heat.  Beyond  this,  we  have  not  succeeded  in  developing  anything  reliable 
in  the  way  of  accelerated  aging. 


APPENDIX  C. 
TABLE  OP  SPECIFIC  GRAVITIES. 

Minimum       Maximum 

Acetone    0.797 

Aluminium   silicate 2.61  3.02 

Aluminum  flake 2.56  2.65 

Ammonium   carbonate    1.50  1.60 

Aniline    1.00  1.03 

Antimony,  red  2.87 

golden  2.57  2.90 

crimson     3.11  4.20 

black   4.80 

Arsenic  yellow  2.75 

Asbestine    2.60  2.82 

Asphalt   0.99 

liquid    0.99      - 

"         Trinidad 1.20 

Balata   1.05 

Barytes  (blanc  fixe)    450  4.92 

Beeswax    0.97 

Benzene 0.745 

Bitumen   1.07  1.16 

Black  substitute 1.10 

Bone  black   2.20  2.32 

Brown  substitute 1.07  1.32 

Burgundy  pitch  1.10 

Camphene   0.865 

Candelilla  wax    0.99 

Carbon  bisulfide   1.26  129 

Carbon  black  1.68  1.89 

Carbon  tetrachloride   1.61 

Carnauba  wax   0.995 

Castor  oil   0.958 

Ceresin   0.918  0.922 

Chloroform 1.52 

Chrome  yellow,  light  6.41 

"  "        medium    5.73  5.84 

deep    5.91  6.08 

Chrome  green   5.24  5.44 

Clay,  China    2.60 

"       Blue  Ridge    2.55 

"       Dixie    2.58 

Coal   tar 1.05  1.27 

Cork    0.24  1.00 

Cork  dust   1.16 

Corn  oil 0.926  0.930 

Cotton    1.45  1.55 

Cottonseed  oil   0.922  0.93 

Dimethylaniline > 0.958 

149 


148  THE  ANALYSIS  OF  RUBBER 

atmosphere  of  gas  free  from  oxygen,  or  any  gas  which  may  have  a  tendency 
to  react  with  rubber. 

The  most  which  can  be  claimed  for  this  test  is  that  it  is  useful  in  com- 
paring compounds  of  about  the  same  type,  or  of  cures  of  the  same  com- 
pound. With  oxygen  excluded,  it  might  be  extended  to  include  compounds 
of  different  types,  which  are  likely  to  be  exposed  to  the  same  degree  of 
heat.  Beyond  this,  we  have  not  succeeded  in  developing  anything  reliable 
in  the  way  of  accelerated  aging. 


APPENDIX  C. 
TABLE  OP  SPECIFIC  GRAVITIES. 

Minimum       Maximum 

Acetone    0.797 

Aluminium   silicate    2.61  3.02 

Aluminum  flake 2.56  2.65 

Ammonium   carbonate    1.50  1.60 

Aniline    1.00  1.03 

Antimony,  red  2.87 

golden  2.57  2.90 

crimson     3.11  4.20 

black   4.80 

Arsenic  yellow   2.75 

Asbestine    2.60  2.82 

Asphalt 0.99 

liquid    0.99 

Trinidad 1.20 

Balata   1.05 

Barytes  (blanc  fixe)    4.20  4.92 

Beeswax    0.97 

Benzene 0.745 

Bitumen   1.07  1.16 

Black  substitute    1.10 

Bone  black 2.20  2.32 

Brown  substitute 1.07  1.32 

Burgundy  pitch  1.10 

Camphene   0.865 

Candelilla  wax    0.99 

Carbon  bisulfide   1.28  129 

Carbon  black  1.68  1.89 

Carbon  tetrachloride   1.61 

Carnauba  wax   0.995 

Castor  oil   0.958 

Ceresin 0.918  0.922 

Chloroform 1.52 

Chrome  yellow,  light  6.41 

"        medium    5.73  5.84 

deep     5.91  6.08 

Chrome  green   5.24  5.44 

Clay,  China    2.60 

"       Blue  Ridge    2.55 

"       Dixie    2.58 

Coal   tar    1.05  1.27 

Cork    0.24  1.00 

Cork  dust   1.16 

Corn  oil 0.926  0.930 

Cotton    1.45  1.55 

C9ttonseed  oil   0.922  0.93 

Dimethylaniline  0.958 

149 


150  THE  ANALYSIS  OF  RUBBER 

Minimum 

Diphenylamine   1 .16 

Fossil    flour    2.00  2.60 

Fuller's  earth   1.80  2.70 

Gasoline,   72-75   Be    0.700  0.707 

Glass,  powdered  2.49 

Glue  1.30 

Glycerin    1.25  1.30 

Graphite   1.95  2.40 

Guttapercha    0.96  1.00 

Hexamethylenetetramine    1.25 

Indian  red  4.80  5.25 

Infusorial  earth   1.66  1.95 

Kaolin    275 

Lampblack   1.53  1.75 

Lead,    chromate 5.65  6.12 

oleate   1.50 

'       red    8.17 

1       sublimed  blue   6.40 

white  6.20  6.30 

white    6.10  6.75 

Leather  fiber   1.40 

Lime    2.21  2.40 

Linseed  oil  0.94 

Litharge    8.90  9.52 

Lithopone 3.60  4.25 

Magnesia    2.16  3.65 

Magnesium   carbonate,   light    1.74  2.22 

heavy     3.00  3.07 

oxide    3.20 

Mica    2.80  3.20 

Mineral  rubber   1.00  1.06 

Montan  wax  1.04 

Ochre  3.50 

Ozocerite    0.90  0.95 

Palm  oil  0.94 

Paraffin 0.869  0.91 

oil    0.90 

wax 0.91 

Petrolatum  0.90 

Pine  tar 1.05 

Pitch    1.23  1.28 

Prussian  blue 1.96 

Red  oxide   4.82  5.16 

Rosin 1.05  1.08 

Rosin  oil 0.98  1.10 

Rubber,  Accra  flake   1.02 

"        Amber  crepe 0.92 

Assam    0.967 

Benguella 0.928 

"        Borneo   0.916 

"        Cameroon   0.929 

Cameta  0.916 

"        Caucho  ball  0.915 

Centrals 0.93 

Congo  0.93 

"        Guayule  0.975 

extracted   0.995 

"        Madagascar    0.915 


APPENDIX  151 

Minimum        Maximum 

Rubber,  Manicoba   0.93 

"         Mozambique    0.939 

Niger  flake 0.93 

"        Para,  coarse 0.95 

"      fine  0.94 

Penang     0.918 

"        Pontianak   0.99 

"        Roll  brown  crepe 0.95 

Senegal 0.929 

Sernamby    0.918 

"        Sierra  Leone    0.923 

"        Singapore    0.937 

Smoked  sheets 0.91  0.95 

West  Indies   0.935 

Starch    1.50 

Sulfur    1.96  2.07 

Sulfur  chloride   1.69  1.17 

Talc   2.00  2.78 

Thiocarbanilide    1.30 

Tripoli 1.95  2.25 

Ultramarine     2.30  2.40 

Vaseline    0.84  0.945 

Venetian  red    1.96  2.07 

Vermilion     7.89  8.10 

Wax  tailings   1.00  1.08 

White  substitute   1.04  1.14 

Whiting    2.60  2.72 

Wood  pulp    1.43  1.46 

Yellow  ochre    3.50  5.00 

Zinc,  carbonate    4.42  4.45 

"      oxide     5.38  5.60 

"       leaded    5.64 

"      sulfate    3.62 

"      sulfide  3.50 


SUBJECT  INDEX. 


NOTE.    Figures  in  Roman  refer  to  pages;  figures  in  Italic  refer 
to  the  article  listed  in  the  bibliography. 


Accelerators,  inorganic,  42,  106,  200, 
406,  408. 

Accelerators,  organic,  38,  94,  96,  15, 
16,  41,  42,  43,  60,  78,  112,  ISO, 
133,  138,  140,  141,  142,  144t  148, 


j-t/v^      -B.WJ     -«--f-v  j     ^-~f-^j      -m.~fv*  j  *1$     ""•T^'j  V-/ aouil.lv/clj       *  i 

149,  150,  155,  195,  255,  256,  271,      Caucho,  17. 

286,  287,  288,  289,  305,  S2S,  347, 

373,  386,  387,  403,  406,  408,  409, 

429,  430,  433,  434,  435,  462,  491, 

492,  493; 

Accelerators,  ultra-rapid,  41,  59. 
Aging  tests,   146,  147,  17f  59,  188, 

189,  357,  412,  456,  457. 
Aldehyde  ammonia,  40. 
Aldehyde  aniline,  39. 
Aluminium,  119. 
Aluminum  flake,  45. 
Amber  crepe,  20. 
Ammonium  carbonate,  46. 
Analysis,  Interpretation  of,  421. 
Aniline  (aniline  oil),  38,  95,  558. 
Antimony,  Crimson,  47,  47. 
Antimony,   Determination    of,    102, 

119,  10,  18,  75,  76,  172,  173,  174, 

254    299,  345,  355,  356,  365,  368, 

371,  431,  460. 
Antimony,  Golden,  49/7,  377,  578, 

443,  482,  494,  495. 
Asbestine,  46. 
Ash  determinations,  83. 


Calendaring,  141. 

Carbonates,  Determination  of,  101. 

264. 

Carbon  black   (see  gas  black). 
Castilloa,  17. 


Balloon  fabrics,  24,  27,  28,  134,  154, 

336,  419. 

Barium  carbonate,  105,  418. 
Barium,  Determination  of.  104   119, 

418. 

Barytes  (barium  sulfate),  51. 
Bitumens  (see  mineral  rubber). 
Blanc  fixe  (see  barytes). 
Bolting,  140. 

Breaking-down  rubber,   140. 
Brown  pigments,  47. 
Burgundy  pitch,  34. 


Calcium  carbonate  (see  whiting). 
Calcium  sulfate,  47,  49,  119,  494. 


Cellulose,    Determination    of,    158, 

219. 

Cements,  66,  142. 
Centrals,  17. 
Ceresin,  32,  55. 
Chinese  blue,  47. 
Chromates,  108. 
Coagulation,  16. 
Coefficient     of     vulcanization,     25, 

401,  454,  455. 
Compounding,  141. 
Compounds,  Preparation  of  rubber, 

26 

Copper,  43,  99,  171,  181. 
Cottonseed  oil,  34. 
Cure,  Definition  of,   14. 
Cure  tests,  Formulas  for,  25. 
Curing  tests,  24. 

Dimethylaniline,  39. 
Diphenylamine,  39,  96. 
Diphenylcarboimide,  40. 
Diphenylguanidine,  40. 
Dithiocarbamates,  41. 
Drying,  139. 
Dyes,  47,  88,  111. 

Elongation,  146. 
Ethylidene  aniline,  39. 
Extraction  apparatus,  68,  38,  260. 
Extract,  Acetone,  68,  73,  156,  251. 
Extract,  Alcoholic  potash,  71,  344- 
Extract,  Chloroform,  69,  71. 

Fillers,  37,  45,  112,  119,  1,  IS,  114, 

221,  259,  278,  SOS,  309,  S26. 
Formulas,  Calculation  of,  117. 
Fossil  flour,  48. 
Frictions,  147. 
Furfuramide,  40. 


153 


SUBJECT  INDEX. 


NOTE.    Figures  in  Roman  refer  to  pages;  figures  in  Italic  refer 
to  the  article  listed  in  the  bibliography. 


Accelerators,  inorganic,  42,  106,  200, 

406,  408. 
Accelerators,  organic,  38,  94,  96,  15, 

16,  41,  42,  43,   60,  78,  112,  ISO,      ^^  wa,»a 

133,  138,  140,  141,  142,  144,  148,      Castilloa,  17. 

149,  150,  155,  195,  255,  256,  271,      Caucho,  17. 


Calendaring,  141. 

Carbonates,  Determination  of,  101. 

264. 
Carbon  black   (see  gas  black). 


286,  287,  288,  289,  305,  323,  347, 
873,  386,  387,  403,  406,  408,  409, 
429,  430,  433,  434,  435,  462,  491, 
492,  493: 
Accelerators,  ultra-rapid,  41,  59. 


£^lA>WJ.dCvV\S*Oj        C4.AUAC*      A  C*^iV*.j         •*•  *  9        v  w  .  V_/^l.  ^OlJ-Lj        V*-j        W « 

Aging  tests,  146,  147,  17,  59,  188,      Chinese  blue,  47. 


Cellulose,    Determination    of,    158, 

219. 

Cements,  66,  142. 
Centrals,  17. 
Ceresin,  32,  55. 


189,  357,  41%,  456,  457. 
Aldehyde  ammonia,  40. 
Aldehyde  aniline,  39. 
Aluminium,  119. 
Aluminum  flake,  45. 
Amber  crepe,  20. 
Ammonium  carbonate,  46. 
Analysis,  Interpretation  of,  421. 
Aniline  (aniline  oil),  38,  95,  358. 
Antimony,  Crimson,  47,  47. 
Antimony,   Determination    of,    102, 

119,  10,  18,  75,  76,  172,  173,  174, 

254,  299,  345,  355,  356,  365,  368, 

371,  431,  460. 
Antimony,  Golden,  49,  7,  377,  378, 

443,  482,  494,  495. 
Asbestine,  46. 
Ash  determinations,  83. 


Balloon  fabrics,  24,  27,  28,  134, 

336,  419. 
Barium  carbonate,  105,  4^8. 
Barium,  Determination  of,  104,  119, 

418. 

Barytes  (barium  sulfate),  51. 
Bitumens  (see  mineral  rubber). 
Blanc  fixe  (see  barytes). 
Bolting,  140. 

Breaking-down  rubber,   140. 
Brown  pigments,  47. 
Burgundy  pitch,  34. 


Calcium  carbonate  (see  whiting). 
Calcium  sulfate,  47,  49,  119,  494. 


25. 


Chromates,  108. 
Coagulation,  16. 
Coefficient  of  vulcanization, 

401,  454,  455. 
Compounding,  141. 
Compounds,  Preparation  of  rubber, 

26 

Copper,  43,  99,  171,  181. 
Cottonseed  oil,  34. 
Cure,  Definition  of,   14. 
Cure  tests,  Formulas  for,  25. 
Curing  tests,  24. 

Dimethylaniline,  39. 
Diphenylamme,  39,  96. 
Diphenylcarboimide,  40. 
Diphenylguanidine,  40. 
Dithiocarbamates,  41. 
Drying,  139. 
Dyes,  47,  88,  111. 

Elongation,  146. 
Ethylidene  aniline,  39. 
Extraction  apparatus,  68,  38,  260. 
Extract,  Acetone,  68,  73,  156,  251. 
Extract,  Alcoholic  potash,  71,  344. 
Extract,  Chloroform,  69,  71. 

Fillers,  37,  45,  112,  119,  1,  IS,  114, 

221,  259,  278,  SOS,  309,  326. 
Formulas,  Calculation  of,  117. 
Fossil  flour,  48. 
Frictions,  147. 
Furfuramide,  40. 


153 


154 


SUBJECT  INDEX 


Gas  black,  48,    106,  137,  292,  380, 

381. 

Glue,  36,  108. 
Graphite,  50. 
Greens,  50. 
Guayule,   18,  170. 

Hevea  Braziliensis,  16. 
Hexamethylenetetramine,  39,  95. 

Indian  red,  61. 
Iron  oxides,  51,  119. 

Landolphia,  17. 

Lead,    Determination    of,    18,    115, 

290,  359. 
Lead  oleate,  44. 
Lead,  Red,  43,  107. 
Lead,  Sublimed  blue,  44. 
Lead,  Sublimed  white,  44,  359. 
Lead,  White,  43. 
Liebermann-Storch  test,  35. 
Light,  Action  of,  337,  346,  420,  474. 
Lime,  51. 

Litharge,  42,  13,  106,  374,  402,  475. 
Lithopone,  51,  120. 

Magnesium  carbonate,  45,   119,  21, 

105,  301. 

Magnesium    oxide    (heavy    magne- 
sia), 45,   119,  104,  301,  408,  480. 
Manganese,  58,  59. 
Master  batches,  140,  141. 
Microanalysis,  114,  79,  98,  101,  198, 

199,  452,  496,  497,  498,  499,  500, 

501. 

Microphotographs,   114. 
Mineral  analysis,  97. 
Mineral  fillers,  29,  34,  56,  77,  177, 

220,  266.  278,  281,  339,  342,  414, 

442. 

Mineral  hydrocarbons,  31. 
Mineral  oils,  34,  70. 
Mineral  rubber,  30,  31,  70,  71,  39, 

83,  84,  145,  295,  $10,  338. 
Mixing,  141. 
Moisture,  24,  487. 

Nitrogen  in  crude  rubber,  Determi- 
nation of,  24,  364. 

Oil  substitutes,  28,  73,  9,  12,  19,  95, 
127,  129,  146,  253,  340,  436,  448. 
Oil  substitutes,  Tests  for,  29. 
Oils,    Softening,    33,    82. 
Oils,    Tar,    36,    80. 
Organic  fillers,  37,  110,  118. 
Oxides,  Determination  of,  99. 
Ozokerite,  32. 


Palm  Oil,  33. 
Pale  crepe,  20. 
Paraffin,  32,  70. 

Paranitrosodimethyl  aniline,    40,    94. 
Petrolatum,  34. 
Piperidine,  41. 
Piperine,  41. 
Pontianak,  18,  87. 

Resins,  Rubber,  21,  24,  52,  93,  122, 
234,  257,  277,  391,  402,  438,  446, 
477,  478,  490. 

Roll  brown  crepe,  20. 

Rosin,  35,  70. 

Rubber,  Amber  crepe,  20. 

Rubber,  Analysis  of,  11,  19,  31,  32, 
33,  44,  49,  54,  69,  70,  73,  96,  97, 
102,  116,  117,  119,  120,  123,  124, 
131,  135,  136,  143,  160,  163,  166, 
175,  178,  179,  197,  207,  208,  211, 
214,  216,  217,  224,  237,  239,  262, 
270,  282,  298,  306,  311,  330,  332, 
343,  375,  382,  411,  413,  426,  439, 
463,  465,  467,  470,  472,  473. 

Rubber,  Composition  of  crude,  22. 

Rubber  compound,  13,  139. 

Rubber,  Crude,  13,  21,  26,  37,  128, 
184,  185,  218,  422. 

Rubber,  Definition  of,  13. 

Rubber,  Determination  of,  76,  118, 
48,  100,  190,  196,  240,  242,  250, 
304,  4^. 

Rubber,  Difference  methods  for 
determining,  82. 

Rubber,  Direct  determination  of 
(see  Nitrosite,  Wesson's,  and  te- 
trabromide  methods),  3,  25,  26, 
45,  244,  245,  333,  334,  415,  445. 

Rubber,  Evaluation  of,  8.  89,  215, 
360,  372. 

Rubber,  Formula  for,  12. 

Rubber  hydrocarbons,  13. 

Rubber,  Indirect  method  for  deter- 
mining, 81,  350. 

Rubber,  Insoluble  matter  in  crude, 
21,  15,  35,  36,  180,  238,  354  366 
369,  383,  384,  393,  416. 

Rubber,  Nitrosite  method  for  de- 
termining, 79,  4,  5,  165,  193,  194, 
202,  204,  205,  206. 

Rubber,  Pale  crepe,  20. 

Rubber,  Para,    16,   17,  227. 

Rubber,  Plantation,  19. 

Rubber  for  analysis,  Preparation  of, 
23,  30,  296,  476,  486. 

Rubber,  Reclaimed,  27,  66,  112,  6. 

Rubber,  Roll  brown  crepe,  20. 

Rubber,  Smoked  crepe,  20. 

Rubber,  Smoked  sheets,  20. 


SUBJECT  INDEX 


155 


Rubber  substitutes,  28,  272. 

Rubber,  Synthetic,  279,  308,  328, 
481. 

Rubber,  Tetrabromide  method  for 
determining,  76,  JtO,  50,  61,  62, 
63,  6Jt,  65,  67,  71,  147,  161,  164 
167,  169,  209,  220,  223,  228,  229 
232,  235,  243,  246,  248,  273  283 
294,  ^67,  370,  389,  390  392  441 
W,  449. 

Rubber,  Wesson's  method  for  de- 
termining, 79,  80,  92,  424,  425, 
483,  484,  485. 

Sampling,  23,  64,  417. 

Set,  Permanent,  147. 

Silicates,  102. 

Sodium  bicarbonate,  52. 

Sodium  hydroxide,  42. 

Solvents    for   rubber,    81,    85,    183, 

263,  479. 

Specific  gravity,  111,  108,  363. 
Specific  gravity  table,  149. 
Sponge  rubber,  46,  110. 
Spreading,   142. 
Stress-strain  curves,    146,  210,   361 

376. 

Sulfates,  101,  1,  399. 
Sulfides,  100,  7,  132,  264,  399. 
Sulfites,  101. 

Sulfur,  37,  50,  118,  233,  312,  335. 
Sulfur,    Combined     (see    sulfur    of 

vulcanization). 
Sulfur,  Free,  29,  88,  89,  29,  92,  152, 

268,  269,  330,  440. 
Sulfur,  Total,  84,  1,  2,  22,  52,  91, 

94,    99,    110,   152,   159,   162,  176, 

186.  187,  220,  236,  247,  252,  267 

280,  293,  307,  327,  331,  348    395 

398,  407,  423,  439,  444,  458,  459, 

461,  469. 

Sulfur  in  fillers,  83,  93. 
Sulfur  of  vulcanization,  25,  91.  66 

74,  78,  153,  241. 
Sulfur  chloride,  37,  60  90,  182,  233. 


Talc,  52. 

Tensile  product,  145. 

Tensile  strength.  144. 

Tensile  tests,    143,    258,    291,    309, 

352,  410,  453. 
Tensile  tests,  Machine  for  making, 

143. 

Tensile  tests.  Test  pieces  for,  144. 
Thiocarbanilide,  39,  95. 
Thiurams,  41. 
Triphenylguanidine,  40. 
Tubing,  142. 

Ultramarine,  52. 
Unsaponifiable  matter,  73. 

Venice  turpentine,  35. 
Vermilion,  53,  173,  174,  356. 
Vulcanization  by  selenium,  51. 
Vulcanization  by  ultra-violet  rays, 

212f  213. 

Vulcanization,  Cold,  60,  61,  46,  220. 
Vulcanization,  Definition  of,  14,  57. 
Vulcanization,  Hot,  107,  108,  121, 

220,  284,  285,  374,  379,  397,  475. 
Vulcanization,         Ostromuislenskii's 

theory  of,  62,  68,  313,  314,  315, 

316,  317,  318,  319,  320,  321,  400. 
Vulcanization,    Theory    of,    57,    72 

125,  126,  139,  225,  280,  231,  233, 

394,  404,  405,  428 .  430,  464,  466. 
Vulcanization  with  mixed  gases,  20, 

825. 

Washing,  22,  139. 
Washing,  Loss  on,  22. 
Waxes,  33,  82. 
Waxy  hydrocarbons,  74. 
Whiting,  53,  119. 

Yellow  ochre,  53. 

Zinc,    Determination    of,    18,    115. 

290. 
Zinc  oxide,  54,  120. 


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